There's a plug under the driver's side REAR seat. My '94 has the stock amp under there and the plug is located forward, under the front edge of the carpet. Plug has 16-pins, 2 rows of 8 pins. Pin #14 wire going in/out both sides should be colored (BR/LG) BROWN with a LIGHT GREEN tracer stripe. Should be the only wire with these colors.
1 / 9
2 / 10
3 / 11
4 / 12
5 / 13
6 / 14
7 / 15
8 / 16
Using your 12 volt tester, check for 12 volts on Pin #14 while in reverse with the ignition switch on. You can either probe the back of the pin, or just carefully prick through the plastic wire coating with the tester, just enough to touch the wire inside. Make sure the tester ground clip is connected to bare metal (no paint) for a good ground.
If you DO have 12 volts on Pin #14, there's not much else further back that is common to both lamps. There's a wire splice over the the passenger side rear fender well. Think the trailer tow wires splice in here. If splices get wet, they corrode and often lose connection. Happened to my driver's side power seat.
If you DO NOT have 12 volts on Pin #14, then your problem is forward towards the instrument panel.