most of the major pieces can be taken out and tested at the parts store.

as for the rest, I would guess a test light to the wiring.

wire up the ground to the battery like normal but leave the hot off.

then probe each line with a test light hooked to the pos of the battery.

if it lights up something on the other end is grounded.
Just have to find what it is.
The wiring diagram should help figure that out.