Troubleshoot trailer lights.
Troubleshoot your vehicle with a tester
1 Hook up your trailer and connect the harness. Have a helper stand back to see if the running lights, stoplights and blinkers are working properly. If all the lights appear dim or you have no lights at all, check the harness at the tow vehicle. First disconnect the harness and plug a tester (Photo 1) into the receptacle from your vehicle. This tester will tell you immediately if you have a problem in the wiring from your car or truck. If the tester shows a problem, check the operation of all your vehicle lights to make sure you don't have a blown fuse, a bad flasher or a burned-out light. Clean the connector (Photo 2) and check again. Check for broken wires near the connector. If you still have problems, make an appointment to have the wiring checked. If the lights on the tester function properly, the problem is the trailer lights or harness.
Clean both connector plugs
2 If after you check the tow vehicle for problems the trailer lights still appear dim or aren't lighting at all, clean the connector plug. Spray the connector with an electrical contact cleaner and use a fine wire brush to clean the contact pins.
Check the ground
3 Next, check the ground wire. Most problems occur here. Remove the ground screw and sand the wire terminal and trailer chassis contact with sandpaper. If the ground screw itself is corroded, replace it with a new screw.
Replace bad bulbs
4 If you have one light out (either running lights or blinker/stop lights) you probably just need to replace the bulb. Remove the lens, replace the bulb and check it. If it works, you're on your way.
Clean corroded sockets
5 If the light still doesn't work, you may have corrosion in the socket at the contact points. Attach a bit of 220-grit sandpaper to the butt end of a 3/8-in. dowel with hot glue. Clean the contact points by spinning the dowel and moving it side to side. Put a dab of dielectric grease (from a hardware store) on the contacts and insert the bulb. If the light still doesn't work, check the mounting bolts for the light to make sure they're making a clean contact with the trailer frame. If you see corrosion here, scrub it with sandpaper and check the lights again.
Run a continuity test
6 If the lights on one side still aren't working, you may have a break in the wire. To test for a broken wire, check the color of the wire going to the socket and then find that wire on the connector in front. Clip one end of a long jumper wire (shown) to the connector pin and then clip the other end to the continuity tester. Probe inside the socket with the tester (you'll see the color of the wire that connects to the socket inside). If the light fails to go on, trace the wire and check for bare spots or breaks. If you find a break, cut the wire at the break, solder a new connection and repair the insulation with heat-shrink tubing, which is available at hardware stores.
Replace the wiring
7 If all else fails and you're finding badly corroded parts, you can buy a new wire harness for about $20. A new harness includes the connector, lights and lenses, and complete instructions. In most cases, it can be installed in about two hours.
Here are two sources of wire harnesses and testers:
* www.northerntool.com (800-221-0516)
* www.hitches41ess.com (888-326-0054)