I have a '92 SSEi with the 8-speaker premium sound with the gain control. I was curious as to which speakers does that external amp power? What speakers does the internal amp in the HU power? If anyone had the wiring schematics that would be even better. Thanks

The rear 6x9s have 2 coils in them. The in dash head unit powers the front speakers and one of the coils in the 6x9s, and the external amp powers the 2nd coil of the 6x9s, so you can boost the bass with the gain control. If you were to put in an aftermarket head unit you would wire the head unit up like normal, and then disconnect the external amp because it will not be needed.