You owe it to yourself to check out PIC's. The PicAxe is so easy to program using just the serial output of your computer.
Check out these sites;
I just started working with them over a year ago in this thread;
OldSpark and I had about the same discussions in that thread as in this one.
With a PIC you'd be changing code instead of circuits. You could take whatever's coming out of your car for dimming and program the PIC to give you the results you want.
It really was fun to figure out the programming. You start simple and just build from there. There's lot's of forums that have lots of examples that you can adapt to your needs.
I have my little circuit hooked up to my carputer permanently. I can reprogram it whenever I want.
Seriously, if you were building switching power supplies for fun you'd really enjoy PIC's.
You should just get a few to play with. As you play with them you'll be flooded with ideas.
This is the code I'm currently running on mine to follow light changes.
It's reading a light detecting resistor to adjust the PWM to the backlight.
symbol LED=2 'Output 2 - LED Backlight (pin5 on chip)
symbol LDR=4 'Input 4 - LDR with 10Kohm resistor (pin 3 on chip)
for w3=1 to w1
if w2>=1000 then:exit:endif
if pin1=1 then pwmout LED,off:goto running:endif
if w1<30 then:w1=30:endif
if w1<w2 and w2>30 then:dec w2:endif
if w1>w2 and w2<1000 then:inc w2:endif