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.

Corrosion probably acting like a resistor. I would wiggle every bulb and every blade connector you can find in the circuit.

Corrosion

I personally would start at the fuse holder. Pull the fuse, clean both the holder and the end of the fuse with a wire brush and apply some dielectric grease on both sides.

I'd also use the multimeter to check from ground to both sides of the fuse.

If that's OK, recheck your volts at the light, if you have 12v your good to go.

After that, I would check the back side of the fuse. Disconnect battery and indo the two Phillips screws and lower the panel and inspect. If it's crappy, you may want to go thru the connections one at a time, clean and apply dielectric grease to each of the connectors. In fact, that may be the best way to avoid some electrical gremlins in the future.

You never know what a previous owner stashed above the fuse panel. Early 80's I found a bag and a pipe up there

I shoulda known.

Thanks

Grounds, too. Check those, or just clean them because it's rarely a bad idea to clean your grounds...

--DD

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.).

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.

Most likely culprit in this case are the connections between wires. This includes spade connectors at the fuse panel and anywhere 2 wires come together with some sort of connector including grounds.

There is a spade connector on the back of your fuse panel. You can only get to that by dropping the panel. If you do drop the fuse panel, disconnect the battery negative first until the panel is moved to where you can do some measurements, then reconnect the battery negative. Same thing when putting the fuse panel back in place.

I just went through the same. After doing the above recommendations, I used a small amount of di-electric grease on the connectors. Esp the lick plate and rear lights where moisture is an issue.

Thanks, all. Cleanup is in progress. The wires to the relays on the back of the fuse panel make it difficult to get to some of the connectors. How hard is it to reinstall the relay sockets?

Stick the rubber nub into the hole. Grab with needle nose pliers. Use a rotating pull motion and it should pop in. A tiny bit of dish soap might help.

Thanks, Mike.