I plugged in trailer lights now the running lights on my 1998 1500 Gmc truck don't work. Could it be a relay? Fuse seems to be ok.

The T PARK fuse in the rear fuse-relay block supplies battery positive voltage to a separate park lamp circuit connected to the trailer wiring harness..
For trailer wiring, separate turn signal circuits are connected through the GMSF to the trailer wiring harness.
Fuse Block - Rear
Beneath the left rear seat
Trailer TR/PARK fuse 42 10amp

Check the plug at your gmc outlet, your right signal wire has been damaged before the plug somewhere. If you have a test light switch your right signal on and make sure it is working at the plug on your gmc, you will find it is not working, than trace the wire back from the plug till you find where it has been damaged or you can run a wire straight from plug to your right signal.

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

Check fuse 28 in the fuse block under the left side of the dash. There is also a relay for the trailer park lamps in that same fuse block. The fuse and relay are the trailer parking lamps and run independantly of the vehicles parking lights. You can also try backprobing the other trailer plug (you should have one 7 pin and one 4 pin) to see if there is power at the park light pin. If there is power at the other trailer plug than the fuse and relay are bad and you may have a bad/corroded plug or bad wring to the plug that doesn't work.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

The fuses for the trailer tow adapter are in the normal fuse panels.

Engine Compartment fuse panel
101 - Is the battery charge circuit ( hot in run position )
20 - Left Stop / Turn fuse
19 - Right Stop / Turn
5 - Trailer tow reversing lamps & Trailer Tow parking lamps

Cab Fuse Panel
23 - fuse for the relay for the trailer tow battery charge circuit

All of these are in the owners manual

If you did not get an owners manual with your truck ( purchased used ), you can download one from Ford :
https://www.fleet.ford.com/maintenance/owners_manuals/default.asp

Did you check the fuse for the trailer tow adapter paring lamps ? This is a separate fuse powered by a relay, it is not the same circuit as the truck's parking lamps.
This is fuse # F10 & The half size ISO relay, R201 in the fuse panel in the passenger side kick panel.
This is based upon being a new body style 2004, not the 2004 Heritage or Lightning models, those are different fuse & relay locations.

Strange as it might seem, I had a similar problem with my 2003 Sierra 2500HD. All lights on the truck worked fine, and all lights on the trailer worked fine with the exception of the right brake light. I chased all the obvious possible reasons, (blown fuse, shorted wiring, etc). It turned out to be a sticky relay in the flasher unit in the dash. It is located behind the fuse panel on the left side of the dash, and is accessed through a hole behind the lower panel under the steering column. I removed the fuse panel cover and the lower plastic panel under the steering wheel and was able to remove the flasher unit. I ordered a new one but before I installed, it I checked the old one. The plastic cover comes off easily. There are two small relays for the trailer lights and the one for the right side was stuck in the off position. I freed it up, using a toothpick, gave it a spray of conatact cleaner and re-installed it. It is working fine now. I kept the new flasher unit as a spare in case the problem arises again.

Your problem may not be the same as mine, but at least you will know where the $100.00 flasher is located!

Hope this helps,

Hooty

check trailer ground

it seems you have a short in either one of the lights or interlock. all are connected to the same fuse. trace the wiring from the fuse box back to the component. Using a test light test in between the wires. check for any obvious breaks in the wiring.

It is in the fuse box under the hood on the drivers side