One thing we must remember is that there are lighting technologies that are 'dimmable' and others that are not.
CCFLs for example... you just can't attach a rheostat and use it to dim the light. But let's assume we are talking of LED technology. LEDs are dimmable.
I used 'calibration' to refer to the task of adjusting the photocell so the whole install ends up producing the desired brightness level at midday of a sunny day and also producing the desired brightness level at midnight on a moonless night. My guess is that achieving those results wouldn't be trivial.
Why would one want automatic dimming? Just because I'm lazy and don't want to have yet another control knob in my car life's too complicated as it is.
Newer models don't seem to have this automatic dimming nor need it, so perhaps just attaching a resistor somewhere in the circuit would be fine. I'll try some of that and post results here for you guys to judge if it is worth the minimum effort it would take.
BTW, congrats for being just 16 and already having realized there are bad cars, good cars, and bimmmers, E36 bimmers to be accurate.