My 2000 chevy silverado 1500 4.8L has no running lights on my trailer when I plug in the 4-way flat harness. If trailer is plugged into another vehicle it works fine. Problem started when 4-way flat with inline circuit tester from truck side had to be replaced. We put back the wires the same way they came off. At first they worked, the battery gauge bounced around when the turn signals were used but we had no running lights. Then, it was discovered we had running lights on when we were in reverse gear. Now, there is no lights at all and the new 4-way plug on the trailer with inline circuit tester has no lights on it, so I'm guessing there's no power. I already checked all fuses under hood, inside driver side door and relay by pedals. I also checked the ground under the bed, by the neck filler, on frame by fuel pump, the one to the block and on the front bumper. All grounds were clean, in tact and secure. I dont know much about electrical stuff but otherwise pretty handy. Can anyone help this girl out, please? What am I missing? I cant keep pulling my trailer and drive with the hazard lights on because its wearing out something by my instrument panel up in the dash/ steering column. I can hear little gears grinding when I have the hazard lights on for a period of time. Besides that, there is no hazard lights working on the trailer anymore. Thank you in advance.

Rachel:

something is mis-wired somewhere... a trouble light or multimeter would be a handy thing to have right now. The four wires coming out of the plug are 1. ground. 2. left turn signal. 3.right turn signal. running lights. Ground the trouble light clip to some metal piece of the truck (the frame is best... make sure that the metal that you connect to is not painted or rusted. fire the truck up and turn on the headlights. Probe each of the connector contacts. the one that lights up the trouble light is your running lights on the truck's connector. Do the same with each turn signal. Left turn signal on... check the other three contacts in the connector. The trouble light will flash with your rear turn signal. Same thing with the right turn signal. Write down the colors of the wires and note which color corresponds to each function. Use your jumper wires to test the trailer plug. Connect the negative pole of the battery to the frame of the trailer with the negative jumper. Hook the positive jumper to the positive pole of the battery and connect a thin wire to the other end of the positive jumper. that thin wire will be positive or hot.... do not touch any grounded part of the truck with the thin wire... that means chrome trim, frame, or the ground wires from the plugs.
Now check to see which wire from the trailer plug is grounded to the trailer with a screw or nut and bolt. Do not touch that contact with the thin wire or you will blow a fuse! touch each of the other three contacts one at a time and observe which trailer lights comes on. Note which wires operate which lights. You will probably have to switch some wires where they hook up to the truck. For example, when you touch the blue wire contact with the thin wire and the trailer left turn signal illuminates, the blue wire to the truck needs to be connected to the left turn signal wire on the truck. Same with the right turn signal and running lights. Also check to make sure that the ground wire from the plug is a clean, tight connection at the truck and the trailer. sounds like a lot of work, but that is the only way that you will know that the rig is properly wired.