It should be relatively obvious when you crack open the case to the headunit where it is. You probably won't be able to seperate it from the main circuit board. You should see some relatively large caps to smooth out the high notes, and then probably quite a few transistors not in surface mouint DIP packages but the 3 pin black head ones. Easiest way to find out I think would be to trace the speaker connects back to around this area, and then hook the HU up to a power source and try to feed sound of some sort through the circuit and see where it is when sound shoots out the other end amplified! Of course do that only after you have a pretty good idea where it is, and are just getting to find out which is which channel and ground/positive supply.