Trailer brakes stop working when headlights are on

Try connecting to a different trailer and see if you have the same problem.

i had a similar problem on my Chevy 2003 2500hd.

There's a flasher control module that went bad and exhibited a similar issue.

When I stepped on my brake lights, the right turn signal didn't work. When the clearance/running lights were on the right turn signal wouldn't flash.

I just replaced the control module and the problem abated.

Just something to think about...

Sorry, for not providing any updates as I was in the process of moving my daughter into college and getting her settled in. I tried several different testing methods including looking for a bad ground, connecting a separate ground from trailer to truck, hooking up another truck to see if it had a similar issue and even having a mobile RV tech come look at the issue. No luck! Appears it may be an issue with the IBC itself.

agesilaus wrote:
A short somewhere in the wiring? I just spent an afternoon patching my cable after it got some road rash

Love answers like this (knee jerk) because.

NO it is not a short.. it's an open.. ground

YOu have a poor/bad ground return between trailer and truck. I've had to improve a few of the ground over the years.

The ground can handle current from Brakes OR lights but not both.

TO TEST (yes there is an easy test) you need nothing more than a SINGLE (black lead if you have the choice) automotive jumper cable Though any decent jumper cable will work

Hook one end to any non-painted part of truck Example a heavy bumper if not painted. (you can use test lights to verify good ground )
The other end to any non-painted part of trailer (Frame or tongue)

Then see if it works right. if so. improve ground.

Your 7 pin SHOULD have a ground wire that connects both the Truck and the Trailer. IF NOT, then the 5th hitch plate is the ground between the 2, which is NOT adequate. Do NOT connect the trailer to the truck. just have the hitch off the plate and then connect the 7 way and see if you get tow lights and brake lights and brake operation. You can move slightly a foot or so to see if the brakes grab. That will determine the ground situation without testing the 7 way plug. Usually the ground from the Trailer end is secured to a screw in lug in the 5th wheel hitch tongue. Doug

Sounds to me like the Brake Controller is wired incorrectly.
Maybe the Ground Wire of the Brake Control Module is connected
to the wire that goes to the Park Lights.

With the Park Lights off the Brake Controller gets its
0 VDC through the park light bulbs to Ground.

When the Park Lights are turned on there is no
0 VDC for the Brake Controller.

Short to blue tow vehicle lights would go wacky
Short blue to the negative breaker or fuse operates

I miss read the original post. Read as if it was brake lights and not the electric brakes.
Does sound like the system is backfeeding. It could be ground or a short between the brake (blue) wire and the tailight wiring.

A restricting negative is easy peasy to diagnose

Turn on parking lights, then actuate the manual brake lever on the controller. What happens to the lights on the trailer?

If the circuitry goes "Bates Motel" with the lights on the trailer you gotcha problems with the chassis negative.

Start the exam process by opening the plug and look for the usual suspects.

Try pulling the brake lamps in the trailer. It could be a bad filament shorting to the tailight filament.
Test trailer with jumper wire from trailer battery to light functions. Look for anything odd in lighting.