2001 f 150 extended cab. trailer lights were working. had to replace a trailer light uhit because it was broken. now lights do not work properly. running lights work. turn signal only works if running lights on. only one side will work at a time. ran ground to both sides, only one side will work properly. can get get both sides to work together. tapped into truck harness above oem plug. still does not work properly. tried a new light kit. same problem.

The description of the problem is very confusing. If the trailer is disconnected, do all of the functions work OK? If so the problem is with the trailer wires. If all worked before you replaced the light, then thats where the problem lies. The turn signal/brake lights are a seperate circuit than the running lights. The brown should be hot with running lights, and the yellow and green are the turn/brake signals. The trailer lights should be grounded to the trailer frame, and the frame connected to the white wire, Make sure the white wire is connected to the plug on the truck side and connected to the frame of the truck. The turn signal that works with only the running lights on doesnt make sense, and grounds are the most common item. Some Ford trucks have seperate fuses for right and left. There a small fuse box under the hood, near the brake booster.