Wiring issues?

Looking for some ideas on what to check. My 98 chevy truck has an issue with the brake lights. Everything is fine until I hook up a trailer (any trailer). Everything checks fine with no trailer plugged in. The problem occurs when I plug in a trailer. Hazards lights work fine. Turn signals work fine and the brake lights come on when the brake pedal is depressed… the problem is once the brake lights come on, they stay on, both the truck and the trailer. They won’t go off until the trailer is unplugged.
This truck also has an auxiliary brake controller. It was used to pull a camper in it’s previous life. The trailer brake wires are not connected to the truck receptacle, in fact it just has a flat, four wire connector right now. I’m going to delete the brake controller option and hopefully that takes care of the issue?

I used to have a Suburban a few years newer than that and a few times the flasher module went out and it impacted the trailer lights. Try replacing that. Likely available at most auto parts stores and plugs in under the dash.

I would think it has to be an issue with the trailer adapter. Does the brake light on the cab stay lit too?

I disconnected the auxiliary brake control. They also had a hot wire running back to the plug, along with the blue wire for brakes. Hooked up my trailer and checked turn signals, hazards. All good.
When I tried the brakes, they stayed on. Found out that if I turned on either turn signal, then the brake light on that side would go off.
I figured that might be a way to get me by for a couple short trips, hauling the mower down to the farm. I could just bump the turn signals or the hazards each time after using the brakes. So, I’m cycling through this scenario when I turned on the hazards and they didn’t flash. I go back and the truck taillights are stuck on again but this time, no trailer lights. Only way to get the truck taillights off is to unhook the trailer plug… and there is a blown fuse now for the trailer lights. (I remember that from a session a couple years ago)
Ran out of time and patience!

A trailer fuse on a 1998? The fuse panels on those were the same as my 96. I would crawl under the rear of that thing and follow the trailer wires to see what device is there and see if it has some external power feeding it. I had some weird issues with my lights without a trailer due to wiring harness adapters for the trailer getting cracked and shorting out. I could hit the brakes and my dash would light up, replaced T harness adapter and all was good for a couple years until it started blowing fuses for the running lights, after that I soldered a four wire plug in and used heat shrink on the wires

Haven’t had a chance to check for that fuse yet… not sure when I’ll get to it, or if there even is one? Probably my feeble mind playing tricks on me. Might have even been something on my previous truck? Thought about ordering a couple of Diodes to eliminate any backfeed.

Update on the work truck…. I finally decided to get serious on this trailer light issue. Just picked up an ALUMA trailer and I want those pretty LED’s to light up!
After tearing out the Brake Controller and disconnecting all the wires I could trace down (up under the dash)… I still had the problem of truck tail lights staying on once I hit the brakes… or turned on the flashers. I rigged up a couple of spare trailer lights, plugged them in and everything worked perfect. That’s when I decided to connect the ground wires. Yep… same problem! So the ground wire coming thru the harness (white wire) was no really a ground wire. I don’t know what it is for? Don’t really care. I just grounded my trailer light harness into the frame and everything works as it should! Thanks for all the suggestions!
Attached is a photo of my test set-up

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Well done! You earned your solution. Grounding is at the core of many issues.