Lights But No Blink

X2 on the flasher.. It happened to me on our 2004 Ford E350. The flasher unit was under the dashboard behind the steering wheel. A small metal can plugged in to a cable hanging from above. I bought a new solid state one for less than $20 and it immediately fixed the problem.
signal light flasher: https://www.acklandsgrainger.com/en/product/p/TRIHD12?cm_mmc=PPC:+Google+PLA&ef_id=CjwKCAjw2cTmBRAVEiwA8YMgzcGEI1bGW8Of3KK8FK5W1Mdj40v3XJ4UZ9nAYbaNV1u6sWzFzNrebxoCSDkQAvD_BwE:G:s&s_kwcid=AL!3645!3!303439922072!!!g!542982169582!&gclid=CjwKCAjw2cTmBRAVEiwA8YMgzcGEI1bGW8Of3KK8FK5W1Mdj40v3XJ4UZ9nAYbaNV1u6sWzFzNrebxoCSDkQAvD_BwE

If all else fails, consider the flasher unit. This happened on my old 2003 Trailblazer. It was the flasher unit that went bad.

A couple of folks have already ked if the brake light worked. If a standard bulb there should be two elements in the bulb. One for running lights and the other for either brake or turn signal.

Nothing else yank out the bulb and hold it up to the light and see if both elements are in tack, or you could check it with a volt ohm meter for element serviceability.

go with the simple things first.

Let us know if some of the simple tests worked out.

In my experience, the most common cause of trailer light issues is a bad ground. Does the trailer have a dedicated ground in the wiring, or is it using the ball?

Did you check the bulb? I had a trailer where both brake/blinker filaments went out at the same time... Spent an hour checking wires, fuses, ground, whatever, then thought to just get another bulb and try it.. BINGO! Replaced both bulbs and all good... Worth a try anyway.

Good luck!

Mitch

bad bulb

Did the brake light on that side work?

If it is the truck then it means a blown relay, maybe not on GMC but certainly on Fords. Unplugging the trailer with the truck running can cause these relays to be fried.

The tester is essential to debug these issues.

I bought one of those testers for my trailer lights. Plugs into the truck and tells me if any of the feeds are bad. At least this weeds it down to the truck or the trailer. If the trailer, then I pull the light cover and check the bulb. If the truck, well, it's never been the truck.