A long jumper cable with alligator clips at each end, and a multimeter can solve most problems.
- To see if there is power at a point, run one end of the jumper cable to a good ground and use the multimeter to check for DC voltage between the point and the other end of the jumper cable.
- To see if there is a bad ground connection, run one end of the jumper cable to a good ground and connect the other end to the questionable connection. If the device starts to work, you know the old ground was bad.
- To see if two wire ends are connected (e.g., one in the trunk and another under the dash), connect one end of the jumper cable to one end and use the ohm resistance reading on the multimeter to check the resistance between the other end and the jumper cable. If the resistance is high or infinite, the two ends are not connected. If the resistance is very low, they are connected.
The most common problems with lights on fiberglass bodied cars is bad ground connections. I'd check that first.