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.

[screenguy914]

Just to start off, confirm the battery cable connections to the battery and ground are clean and tight.

Use a voltmeter to probe the offending circuit beginning at the fuse box. Input and output source at the fuse should be source voltage (same as at the + post of the battery, typically 12 volts.

From there, measure source voltage along the path to the dim light bulb and see where there's a voltage drop/loss (e.g. circuit connections at the switch, relay, any splice points, M/F harness connections, etc.). Once you find the offending section, examine it for loose and/or corroded terminals and/or loose connection points. Repair, retest, then continue the search if necessary until the entire current path is confirmed okay. At that point, there should be full source voltage available at the load (bulb, motor, etc.).