And by the way the "simple series resistor" would be way greater nightmare to design, even ignoring the corrosion problems.
\First 2x 100W dissipation is ridiculous power to handle in the crammed car environment without forced cooling. Then deal with the failure of such cooling.
Unlike 1mOhm MOSFETS, a lamp short circuit won't just blow the upstream fuse, because the resitors would limit the current (to about 45A, so not that much above the 20A normal working current of each lamp). But it would cause them to generate about 10x as much heat than in normal operation, so a short circuit would be an event really asking for severe car fire.
So no, something that may seem simple can really very quickly become unsolvable nightmare.

Nothing unsolvable. Ballast resistors can be mounted on the lamp rack externally. Some kind of thermal cutoff like a thermal fuse can be added. Still much easier than using a bunch of complicated power electronics and worrying of interference generated. 100W dissipation may look huge in a scale of a small home appliance, but is really minute in a scale of a large car...

But I still think 40+A consumption can be problematic, electronic dimmer or not, by taxing car's alternator and taking almost all charging current available, especially at low RPMs. 

We are talking about cars. There are quite strong restrictions on what you can and can not do. One example it can not weigh half a ton. Other there events like car crashes you have to address. There is rain outside. There is potentially combustible dry organic mess falling onto the car you should not set on fire.
You can not have any open live voltage. Small collision and you have a potential short circuit and so fire.

Today (since 2000's) the car alternator is designed to deliver more than 100A even on the very small basic models, more often it is in the 200..400A range, half of that should be available at idle. This is one of the reason for the dimming: To also reduce the current consumption when the full power is really not necessary. And that means just alone a good reason to not use any energy wasting dimming - with resistors the current consumption would be about 30A, but with this PWM just 20A. I would assume the full power will be used only at higher speed when the engine has sufficient rpm anyway, or a special occasion for a short time.

But otherwise yes, 2x250W is ridiculous power for a car and it may need an alternator upgrade, mainly if it is going to be used for longer periods of time. Running alternators constantly at more than about 30..50% of their sticker current rating is running them very hot and so will have impact on their lifetime.

I would take a good old IR2153 to do this kind of project. Heck, even Helvar used these in their dimming fluorescent ballasts)

The problem with these ready made drivers is, they do support alternate switching, only few support both channels off at the same time, some may support both on (those for current fed boost push pull converters),
but here we need all three modes within one system, both transistors on for the full power, alternating for the "50%" and both off for (fault) shut down. And it is the combination of all three modes within one driver system what is unobtainable with ready made IC's.
So you either make something really from scratch (my approach), or select some IC that could be "bent" to do some of the features and then attempt to attach the missing functionality externally by some glue-on circuit (the way Ash approched it) or implement the functionality into a microcontroller (the way these things are normally done in the industry, as it offers to make the function really matching what the application needs yet it yields the simplest HW so cheapest high volume production).
Each of these ways bring their complexities, neither is wrong, just all of them are somewhat complex and the point is to find the simpler one, also taking into account the capabilities of the one who will be building it.

Well OK, with IR2153 and likes, and by slightly optimizing the schematics, you can do exactly what Ash variant does, at about 1/5 of the parts count

You need shutdown? OK short the timing capacitor to the ground. You need both mosfets on? OK, feed the gates +12V directly with a pair of diodes, or just cut off IR2153 ground as a rough hack, that will do, too)

UVLO? No need to mess with LM2903 and a ton of discretes, the circuit is already on the chip. I'd say Ash circuit is nominating for a Guinness for the most 2N700X transistors used in a small project, though I sure have seen the other contenders

 



   

By the way I was just quickly scanning the MOSFET offerings and it seems the mOhm FETs at 60V are still available with low Vth (logic drive) variant, e.g. two parallel IRLB3036PbF and there are many more.

To me this makes the microcontroller way simpler than I originally thought, the 4.5V minimum gate drive means no need for extra gate driver complexity, at 100Hz the microcontroller has plenty of drive power to drive them virtually directly, just via some protection RDR network:
To prevent latchup excitations from some Cdg coupling - one 100Ohm resistor from the MCU to a "point", from there another 220..470Ohm resistor to the MOSFET gate and a Schottky diode with anode on GND and cathode on that "point". When a negative edge apepars on the drain and couples via Cdg as a negative spike on the Gate, the Schottky clamps the "point" to about -0.3V, which first is below the 0.7V needed to activate the parasitic bipolars (and so eventually the latchup) within the MCU IC and seconds even if it exceeds it, it limits the current to the output so it won't even approach the threshold triggering the latchup. Bteween Gate and SZource then nees to be about 47kOhm bleeder, ensuring the MOSFET stays OFF should the  MCU not start for some reason.

You can start with Arduino, you just need to replace the 5V regulator with something compatible with automotive (at least 40V input electrical rating, add the filters and reverse polarity diode,...) and reconfigure the fuses to use the internal RC oscillator instead of the default quartz (for a reliability reason - quartz tends to fail with even very small leakages on the board, so a bit of humidity and it would fail). And you should activate the toughest brown out detection threshold (the 4.75V) as the undrvoltage protection, in order to ensure sufficient MOSFET drive.
The temperature can be measured via the MCU itself, or using NTS+resistor divider on an ADC input. Similar for overvoltage protection (use 100kOhm/10kOhm resistive divider to convert the 0..50V on the battery input into 0..5V for the MCU to process, I think the VDD would be a good reference for both).
And obviously set and use the WatchDog properly...

In the code

@Ash @Medved So would Ash's circuit be functional and reliable? I quite like the added protections and considerations in his circuit. but when it comes to PCB level design, I don't quite have the training/skill yet to determine whether it is a functioning unit. I can 100% follow the schematic and build the circuit to see if it works (once I get the components mailed in) but determining it is a circuit that will function in the environment/system it will be subjected just reading the schematics alone... not so much.

I like your circuit aswell @Medved due to it's simplicity, however I am having trouble following the schematic. it seems V1, V2, and V3 are voltage sources? or are those momentary switches? I also have never seen an inductor symbol used for a light bulb before. unless those are actually inductors?

It is a simulation schematic, it was intended to show the generation of the gate control signals in all 3 modes: Shut down (for faults), alternate (for "Half" setting) and full ON (for "Full" setting). It is missing the fault detector comparators (under and overvoltage, thermal protection), these are essentially the same as in Ash's circuit (just the polarity has to be the same)
Vxx are indeeed voltage sources. V2 (a constant 14V) represents the main power from the battery, V3 represents a control signal switching between "Half" and "full" power, V1 simulates generation of the fault shut down signal (coming from the protection comparators).

However both of the "pure HW" circuits are rather complex and are lacking a short circuit protection (they rely on the upstream fuse and sufficient wiring resistance), because that is not that trivial thing once you don't want a lot of nuisance trips. I do not see a simple way to do it without a microcontroller. Beside some way to observe the current (that is the easy part - just monitoring the Vds is enough) it needs to be rather fast responding, but requires time masking (so it won't trip on turn On or turn OFF transients; still doable in few HW components), automatic restart logic (the fun starts), but with some time out (so it won't be attempting to endlessly "restart" into a real short circuit; now we are cooking) and on top of that make it so it won't be fooled by the 100Hz PWM as "manual resets"; now we are cooking on gas), this protection will most likely prevent the fuse from blowing, but each restart attempt into a real short circuit is a stress so it should manage everything right.
In a microcontroller the only thing you need is some Drain voltage monitoring (a Vbe as a threshold is good enough into a digital input) and do all the logic in the code (interrupt to immediately shut down, masking it when OFF and for some time after turn ON, then few timer counters to count the restart attempts; about 50 lines of code in C).
And most important the HW is so simple and can be easily made very dumb so with very little chance of something being wrong, so when something isn't working right, you can update the code easily without the need to modify the HW...