Where would I begin to troubleshoot lack of right turn signal at trailer plug?

Make sure to check fuses.

then most likely you have a broken wire or a bad connector

get the standard wiring design
http://www.narva.com.au/products/browse/wiring-diagrams
Yellow is the left turn signal on pin 2
Check trailer plug and see if correct colors used on trailer plug
Would suggest getting a 12volt probe so you can test the car socket when left indicator is going on - flashing
To check that the correct pin has 12volts flashing left on it.

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.

The flasher module sends voltage for the trailer brake and turn signal on the same circuit. If you have right turn signal than we now that the fuse and wiring are good which leaves the module.

Do you get the same result if you turn on the four way flasher?

The module is located under the drivers side of the dash. You will need to remove the trim panels to gain access.

I can give you more info if you need it.

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

I have the exact same issue. I have also tested my trailer on other vehicles and it works fine. I even purchased a light up tester and found the problem at the harness. It seems like a bad ground to me but I can't seem to find one.

The trailer lights have a common brake/turn wire while the truck has seperate turn and brake lights. I think the problem lies with the device that marries the two for the trailer wiring but I don't know where to find it?

I have seen once before that the fuse box was bad on a 2003 Dodge 3500. I had power in and nothing out. We had to replace the box. Not cheap! Hope it is simple like a loose connection though. I think I had to pull the inner fender on the drivers side to access the bottom of the fuse box. Pain in the ****, but...

do both trailers use same plug off truck? does both trailers have break lights both sides? can you get turn signal from plug on truck? have you chk. ground on that side? most problems like this is a ground.

I'd get a 12vdc test light if you don't have one already & start from plug, and keep probing back from there. It may be as simple as a bad connection under truck where joined into harness.

Do u have a 4,5,6,or 7 way (wire) system? If it's 4 way it allows basic hookup for 3 lighting functions:
GREEN is right turn signal and stop light, YELLOW is left turn signal and stop light, BROWN is license, and side marker lights, WHITE is ground.

5way is same as above, but adds a BLUE wire for brake signal or aux. power. Its used on pop up campers, utility and boat trailers.

6way (rectangle plug) green=right turn signal, yellow=left turn signal, brown=taillight, white=ground, red and blue can be used for brake control or auxilary. Used on small motorcycle trailers, snowmobile trailers, utility trailers.

6way Round Plug, same as above, but blue is electric brakes, and black (sometimes red,) 12v; note:sometimes red/black need to be reversed w/ blue to suit the trailer.

7way adds an additional pin at plug in for 12volt power or back up lights. Whoa! hope that answers your question w/out confusing u. Let me know if u need more info.