Plugging in trailer there are not running lights to trailer

You can buy small adapters that you plug into the car harness connector and allows you to test that the power is being supplied correctly.

In your case, I would suspect that the trailer wiring is at fault, rather than the truck's wiring..

https://www.google.com/search?q=trailer+7+way+plug+tester

.

suggest downloading the trailer pin standard for whatever number of pins your trailer plug has and tracing the wiring from the colors . The standard will tell you what color goes to what light and what pin number it should be on . Also 5 pin and 7 pin are different . You can plug a 5 pin into a 7 pin but the wiring is different .

Also check the trailer socket is grounded at the car body properly. As this can cause lots of weird symptoms.

All late model gms have a seperate fuse for trailer lights in the under hood fuse box. Your truck running lights will still work but no trailer lights

Trailer end; Red=Brake
Brown=Running lights
Yellow=Left
Green=Right
White=Ground
You will need a turn and brake signal box IF, you have different bulbs for brake and turn lamps. If the brake and turn bulbs are one(red)lens than no box will be required. Thanks, Dana

Sorry have to ask. Did everything work before? Does vehicle have one color for blinkers (amber) & another for (red) brake lights?. Is it a flat 4 connection or 7 pin?

check you bulb and there sockets its possible that you burned out one of the two

If you look in the under hood fuse block you will find the rt turn trailer fuse is probably blown. I hope this helps. If you need more help related to this problem or others let me know.
Thank You,
Roy

It sounds like your have may have your brake light and running lights crossed on your wiring harness to the trailer. Good luck and hope this help's

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.