NSS / backup lights question

Not familiar with how those lamps/ sockets are mounted. Could easily be a ground problem. Look this IS electrical 101. Use your head...........do you have power at the sockets?

How to find out? Easy. Key on, in reverse, or with NSS jumpered, pull one bulb and stick your test lamp in the socket.

Is the socket grounded? Easy. Find a good ground, like up in the trunk, away from "underneath rust." clip your multimeter to that, and double check the meter by "stabbing" the remaining probe into an adjacent piece of body metal.

With the meter on "low" ohms, stab your probe at the lamp socket shell. should see continuity, very low ohms

Other ways:

With bulbs installed and powered up, hook a clip lead to a good ground and touch to the lamp shell. Do the lamps come on?

Or clip your test lamp ground clip to a good ground. With power on, stab your test lamp on the reverse lamp shells. Does the lamp light, even dim? If so, you have a ground problem there at the socket.