tillithz, now you are starting to see what I was talking about. Again, I'm not saying you were wrong to say that some cars have the pot's or more appropriate rheostats, but more and more are moving away since you won't need any heat sink for a PWM.
c) only slightly primative pwm circuits would be ideal for a head unit that looks for volatage to dim or brighten but does not take in account the time average stated above. The reason is because more advanced pwm circuits that actually calculate the set high voltage and low voltage (due to a load like an l.e.d or light bulb) would not detect a high load or low load and hence would not be able to pulse. Because the pwm works in the way of once it puts out the desired voltage (the high voltage) it monitors that and turns off, then once it sees it hits 0 volts (the low voltage) it turns back on, all the while its doing this so fast that the human eye is unable to see this on off timing.
I think you might be confused on how fancy the PWM's are in cars. They don't calculate load. Actually (not to back pedal here) but the knob is still a pot. I would guess that almost any variable knob in the world is a pot. But instead of having all the current flow through the knob itself it's just a variable resistance that controls a PWM chip and that just varies the duty cycle of a FET. So the load is not handled by the knob, but the FET. With such a low on resistance and fast switching they would create such little heat. Most don't measure the "load" of the interior lights to figure out a dimmer. They set it up at the factory with good settings.
BUT, and this is the big BUT.... There would still be a load on the vehicles dimmer that even if it was a pwm circuit would be able to function and pulse fine. This is known because when you change your radio for say a Avic-z1 You dont typically change ALL OF THE INTERIOR LIGHTING too, like the instrument cluster.
Well the point is to have the load flow through the switcher not through the dimmer knob. Again, because I don't dumb down my information like Crutchfield or some other posts on other sites... since the dimmer knob is a pot, it is a load and current will flow through it. BUT and this is the big BUT (not my wife's big but) where does the current for the lights actually flow through. The point of a PWM is that it doesn't flow through the dimmer knob (pot) itself. And there is only heat built up in the transition of on to off, and that's so fast that there isn't that much heat, so the parts can be smaller (and cheaper).
I had an 87 Mustang GT and the did use a rheostat in the dimmer knob and it would heat up so much that if you used the fog lights and head lights, it would heat up the switch assembly so much it's internal circuit breaker would go off and your head lights would just switch off for no reason randomly. Heat in a switch assembly is bad. 
So even if you do have a pwm, that would not effect the aftermarket unit that should be able to read the wire anyways because there is still a load that the pwm sees to function, only its slightly less since the original radio no longer is connected that has lights in it. This would ONLY adjust the timing of the pwm, nothing else with regard to the pwm.
And thats still not taking in consideration that the lights in the after market head unit are being controlled by the hu and using the dimmer wire as a singal only or if it actually puts a load on the dimmer wire when reading it, to power the lighting within itself, and if that load is close to the replaced hu load.
They aren't that smart, they just figure out a circuit before the car is designed that the dimmer knob is going to produce a duty cycle of between 50% and 80% and that will look good in the car. They don't care how you will change the lights and current they draw, it's just on and off. There is no load calculator for how the PWM works.
I believe with experience with the avic units that it doesnt just "look" at the dimmer for a signal, it actualy puts a load on it to light the lights that the avic unit has.
Well everything puts a load on everything else, but with my circuit or the Z1, the load it uses to sense the ILL state is so low compared to the lights themselves the value is considered 0 in any calculation.
Sort of when you think of heat built up in a PWM circuit. When it's on the resistance is so close to 0 you can consider it 0. No heat. When it's off, the resistance is infinite, so no current and no heat. It's when it switches that you have to watch, but the % of that is so small that you can pretty much call that 0. Yes there are all loads that you can calculate, but you will go crazy trying to figure out what the current/heat produces for 0.000001% of the time at 6ohms... etc... it's so close to 0, you just forget it. 
Also, I wanted to correct something I stated, not sure if in pm or if here, but there are l.e.d's that can be purchased to replace l.e.d's used in the car for instrument lighting, I mentioned that they could be around 150 mA, but after some reading I have found that this year there are l.e.d's made that only require 20 mA and put out a high amount of light so, You could theoretically light everything with NO MORE then 1 amp
Yah, I wasn't going to go down that road. But yes, you can get quite a bit of light out of 20mA these days. And even so, 1 amp is quite a bit. I guess it depends on the application. If 20mA flowed through your body it would stop your heat and kill you.
So,
YES the dimmer is DIRECT CURRENT and not a.c.
YES technically its pulsed, but can function on a NON pulsed rather simply.
NO, the z1 should not interfere with a pulsed type of dimmer and actually should be able to run just fine connected directly to this wire.
Yep, you got it!
NO, the ductiboy circuit should not be necessary for use of the dimmer switch, with the current electronic circuits available in most cars and most new cars even.
While it should. I don't remember the duty cycle it will accept. My circuit will accept as low as a 30% duty cycle averaged over 1/4 second. If that's less than the Z1 will take, it might be useful. But I doubt anyone would need my circuit to hook the Z1 to a PWM signal.
I think my real point of this was that:
An ILL wire I believe should be just 12V when the lights are on and 0V when the lights are off. If it's anything in between, I guess it should be called a positive dimmer wire.
BUT if you measure "8V" with your nice new digital multimeter, it might be a constant 8V or it might be a pulsed 12V-0V signal and your meter is averaging it and telling you 8V.
I guess it doesn't matter how it makes the 8V, either through a variable resistance (burning off the rest of the voltage as wasted heat) or through a PWM circuit, but it makes a lower average voltage for the interior lights to dim.
What I was saying is that if it's a modern car and you get less than 12V on your ILL or Dimmer lines, switch to AC Volts and see if you get any voltage measured. If you do it's a pulsed signal (more and more common in newer cars) if you get 0V AC then it's a constant lower voltage (more common in older cars).
Not that it really matters in the grand scheme of hooking up stuff. But it's what is happening behind the scenes. Most people will hook it up to whatever and if it "works" they don't care. But that's what is going on.