All lights work correct on trailer until I turn on running lights, at that time when I apply brakes the trailer lights go out, and brake lights do not work but turn signals will still work. If I turn running lights off every thing works fine.

12 volt test light or amp gauge. Trailer brakes use 2 wires... 1 ground and 1 hot. The hot has varied amps to apply according to the controller. 12 volt test light works great. use the white (typical) as your ground and the blue (typical) as your hot. Apply brakes. Light should get brighter as the controller is turned up and dimmer as it is turned down. I hope this helps.

Most trailers use a single bulb system with the bright filament used for the brake and turn signal, and the dim one for running light. Your truck, however, has a separate bulb for brakes and turn signal, thus requiring the use of a logic box to operate the trailer lights. Get one from parts store, install and all should be good.

There are three fuses involved.
There is a 25 amp fuse before the brake switch located in the fuse box under the hood. Then there are two 15amp fuses after the brake switch in the box inside the cabin. One is for the lower brake lights and the other is for the high mount light and the trailer connector.
You would need to know if the 15amp fuse for the lower lights has power when the brake pedal switch is applied.

check for broken or loose wires in plug and what you plug it into. Check the wiring is correct.

Short in wiring,, not hooked up correctly, check ground also, make sure all connected

RB

wires on truckhave a feed ,are they connected correctly??

The things you need to check are the wires from the truck and the wireing to the trailer. If you have a 4 pin flat plug the wires are colored white,brown green and yellow.White is ground,brown is the tail light,green is right turn and stop,yellow is left turn and stop.The brake light switch on the brake peddle needs to be adjusted or replaced to cure your other problem.

If you have amber signal lights & red brake lights, it's 2 separate circuits. You'll need to buy a small converter that you wire into harness to correct that. Not expensive, try autoparts store or RV/trailer sales, should have them.

I take it the blinkers and brake lights work ok when not hooked up to the trailer? If yes, then the wiring to the tow hitch is not hooked up correctly. How many wires to trailer pig tail connection? U can have up to 7 depending on what you tow. Check to make sure these wires are hooked up from the truck to tow plug correctly: 7 wired---
brown-Rt turn and brake light, yellow-backup, white-ground, blue-electric brake, green-tail running lights, black-battery charger, red-left turn signal and brake light. Let me know if u have something other than 7 wire, I'll get the wiring color and source info for u.

Your turnsignal flasher needs to be replaced with a heavy duty flasher.
also, your boat trailor wiring harness is grounded incorrectly or the harness pig tail is wired incorrectly.
this is why when you excite your tail lights and brake lights that your reverse lights come on and the reason that the lights are dimmimg is because your circuits are looking for the correct ground.