I have a 1998 Toyota 4Runner. Recently, I installed a trailer wiring harness and now my tail lights and front marking lights won't work. Everything else seems to be fine. I checked the fuse box and didn't see any obvious issues. I sense removed the trailer wiring harness and still no luck.
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You need a "test light". Then you turn on the circuit you want to wire in, tail lights, brake lights, blinkers and poke around the back of your light sockets until the test light comes on. Then you splicew into that wire and run it to your plug in. If you don't know which wire is what on the trailer, hook it up and with the wire fastened into the truck harness, try it on the trailer harness wires until you get the proper lights working. Process of elimination. Hope this helps.
The tail lights and licence lamp is all the same circuit, should be a brown wire to all 3, it sounds like its broken in the harness somewhere (especially if the front park lamps work, then the fuse and switch are good). Trace back from rear lights until you find power. Check for pinched wires or cut, or bulges in the wire indicating resistence. Also if you have wired in a trailer plug, disconnect it and retry. Trailer wires are very seldomly wired properly and cause lots of light issues.