Just bought a 2018 f150 and have tried to hook up my boat trailer (4 pin connector). The lights all work and the trailer was in working order when attached to my old truck a Ram 1500. Now, the lights all go on when i put the key in the ignition but they turn off once i turn the key. The dash displays trailer attached then trailer disconnected about 1/2 the time i turn the key. Once the key is turned no lights work and i have no indication of a trailer attached. I see no damage to the wires and can't figure out what the issue could be.

 

The brake, taillights and turn signals should always be on so you should be able to measure voltages without the trailer connected. One thing you might try is reseating the connector on the backside of the 7/4-pin connector mounted in the bumper. I've never used the 4-pin but I use to get false trailer disconnects using the 7-pin. While the brakes and aux line would stop working the lights still functioned. The false disconnects were from the connector I suggest reseating.

If you plug in the trailer without the key in the ignition do the taillights lite when you turn on your headlights? Brake lights work when you push on the brake pedal? When you start the truck with the headlights on can you make the taillights come back on by wiggling the connector?