All lights work but one brake light stays on at all times.Where should I start troubleshooting?

Sounds like it's being "fed" from a tailight "source". Assuming that it's not on if the tow vehicle lights are not on.

With a voltage tester, check the "hot" wire to see if it's got full power. Then start checking the tailight unit itself to see if somehow the hot feed to the tailight somehow is hitting the feed to the brake light. Be sure to check the grounds. Bad grounds could drive Thomas Edison nuts.

Electrical bugs can be the most frustrating to figure out. After two years of screwing around, I finally stripped my entire trailer clean of everything, rewired everything, all new "LED" lights, and ran a seperate 12 guage ground wire to each light instead of using the trailer. All connections made with waterproof connectors then six coats of liquid electrical tape on top of that. It's done....

Its on from the moment its plugged to the tow vehicle.Doesn't matter whether the tow vehicle lights are on or off.

Well that certainly changes things. Sounds to me like it would have to be in the tow vehicles's wiring harness. Stupid question please??? Do you have a "factory" plug that plugs into the truck recepticle, then the connector from the trailer plugs into that factory plug or did tow vehicle connector have to be wired in?

If the light comes on as soon as you plug into the tow vehicle, then that one circuit is constantly "hot", which means either that wire is somehow crossed with another wire that is hot all the time (possibly nothing to do with the tail lights) or somehow you are getting a "backfeed" from another source. Electrical problems can be one of the most frustrating things to track down. You're going to have to have a decent circuit tester/voltage tester and just start from the connector and work your way back. Have a couple of cold beers ready, you'll need it.

Vehicle connector was wired in.Dodge dealership sent it to a trailer hitch place before I bought it.Truck 98 Dodge Ram.Trailer is an 02 Southsport.I've had a few cold beers already.Let me look again at the truck side.We traced every wire on the trailer side and everything looks good.Bulbs,sockets etc.Thanks for the help.

Found Draw Tite trailer power module but didn't see anything loose.Its dark here now so I'll put off the troublesearching til tomorrow.Back to the aforementioned beverages.

It sounds like the brake light for that side (also turn signal) is wired to the 12v hot or accessory- the problem is on the truck side if that's the case.

Wannabe

I want to buy Wellby a few beers and watch him play with a Tazer gun!

If the brake light is coming on as soon as you plug the trailer connector to the truck connector, I think it has to be on the truck side.

If the wire has somehow crossed another circuit, a way to at least find out which circuit is this. Get a voltage tester that shows "continuity", then get about 25 ft. of wire. Attach one end of the 25 ft. wire to the brake light terminal that is always hot. Determine if the ignition on the truck is off the circuit is cold. If it's still hot with the key out, disconnect the battery. Then wire one side of the voltage tester to the end of the 25 ft. wire and use the other wire and touch each of the fuse connectors at the fuse box. If the circuit is crossed somehow, as soon as you touch the circuit it's crossing you'll show positive continuity. At least then you know what circuit to look at and find how it's crossed. Hope you understood that babble. Russ