I've got a 1999 F-150 bought new with the factory installed four connector trailer wiring harness below the rear bumper. Hooking up my son's boat trailer to it I've got no lights at all (tail, stop, turn, etc.). Hooking the trailer to his truck, Yukon, all works fine so the problem's not the trailer wiring. All the F-150 lights work fine with or without the trailer harness plugged in. The connector on the trailer is the round type with standard brown, yellow, green and white wires. I bought a four connector flat pigtail and spliced into the wires on the trailer. Being electrically challenged where would be a good beginning to troubleshoot? I can meter it if need be if I can find the meter.
Thanks in advance for the assistance.
Larry