So this happened last year as well, no tails (have turns and brakes just fine). At the time it was in for inspection so I quickly wired a jumper to one of the lights and ran it up to a marker light up front. Lights came on. We then took a trip and I disconnected the "jumper" and the lights worked fine. Here is what it is doing. I cut the wire going by the rear bumper just before it goes up as there was a crimp connector someone had used for trailer lights at one point thinking that maybe it was just getting a bad connection. Well so here is what it does. When the lights are on and the wire is not connected this wire coming from the switch has 12 volts. As soon as you wire it back to the tail lights there is no voltage. There is also no voltage if I just wire it to the trailer lights either. I don't get what is telling it to loose the voltage once it is "loaded" Also tried to run a small test light and nodda. If I run a power wire directly from the battery the lights all work fine. Help me understand this.

You're possibly getting a backfeed from the taillights because this is a ground wire. Touch it quickly to a known ground and see if the lights work (DO NOT touch it for more than a split second!)

Hi, I found this info to be helpful and somewhat easy to understand. http://www.fluke.com/fluke/use...cal/diagnosevoltdrop
Good luck and I hope to read a post about what the fix was.
Mitchell Gentry

Okay still at a loss as it doesn't make sense. I did find a poor ground at the one tail light housing. Didn't change anything though. Are these relayed by chance? I don't have a clue where any of my relays are as I haven't found a replay block on the chassis anywhere so a location on a 86 454 Regal 25 footer would be helpful I would guess. Any other thoughts? I'm not a horrible electrical person btw as I own a TVR with lots of lucas stuff (and of course being fiberglass I have double the fun as each bulb needs its own ground.

Here is a link to the 1986 GM P30 chassis wiring diagram. They show a direct link from your headlight switch to the tail lights.

https://www.barthmobile.com/eve...&s=7031072951&cdra=Y

When Barth added the coach clearance lights they were attached to the GM circuits and are not on the GM diagram. They usually feed from the headlight switch via the rocker switch on the instrument panel.

Be sure your tail lights have good connections in the sockets, and good grounds, or strange voodoo can happen. If sockets are all OK it sounds like a poor connection in the circuit coming from the front: shows voltage when floating but drops when loaded.

good luck.

Thanks, I don't see the grounds or the bulb connectors being the issue as I have my hitch bike carrier on that I put lights on since the bike will block the rear. All that is new and they don't work now either from the wire when the body tail lights are taken out of the mix. They have their own hard wired ground as well. Is it possible that the switch itself was wired to send power to the front parking lights seperately from the tail lights and I just have a bad switch? Again, the front lights are on and working just fine. I now have a jumper running from them to the rear but I am getting quite a bit of a voltage drop. About 9 volts back there under load of 6 1157 bulbs with 14 gauge brand new wire.

With 30' of 14g you will get about a 2v drop with 6 1157 bulbs.

As Craig suggests, 6 bulbs should draw about 6-9 amps depending on which filaments (brakes are brighter than running lights) a 14 gauge wire should handle that with a couple volts loss at most. Whatever is feeding power is wimpy and the load is pulling it down. I would check the connections on the power end of the wire, and switch connections as well. You are on the right track I think.