electrical problem, Where did my volts go? Options

[JimN73]

I have 12 volts at fuse #7, but only 6 at the rear trunk light and the license plate lights (they're all really dim).

The wiring harness looks old but OK, but I haven't taken a really close look.

What could cause that and where can I look?

Thanks for your help.

[somd914]

Corrosion on a single bulb socket would not be the cause this since several bulbs are affected. Keep in mind if a bulb blows, its resistance goes infinite but the rest of the bulbs on the circuit stay bright since they are wired in parallel.

Therefore, the culprit is a loss of voltage prior to the first bulb in the circuit that goes dim, or a bad ground. I'd bet on a bad ground as Dave Darling suggested. If not the ground, you likely have a bad connection or partially broken wire at the first dim bulb in the circuit as screenguy914 suggested. However, cleaning grounding points is easy, and more frequently the cause of such issues, at least from my experience.

Good luck and let us know what you find.