The left trailer brake light doesn't work. The turn signal works fine. The problem was intermittent for awhile but now seems to be constant. All the fuses are good. The yellow wire has continuity from the trailer plug to the end of the rear harness. I suspect the flasher module. Is there anything else it could be?

I would check for corrosion in the back of the plug. Load test the wire to be sure there isnt an intermitten connection. Just because you have continuity doesnt mean you have a good circuit.

There is no corrosion. I have had the entire rear wire harness, including the plug, apart and it is all good. The truck has not been used for off roading and has low miles. However I did launch a boat without unplugging the trailer. I'm wondering if that might have messed with the electronics in the flasher. I have two trailers and everything works on both except the left brake light.

Quick and dirty check? Plug the trailers into another truck. Eliminate them first. I've had a problem on one TRAILER pin that affected more than one trailer, I think the crud from the truck corroded up two trailer pins.

Assuming they all light up, then you know it is the truck.

Do you have juice to the pin on the Bargman truck male side? I find a test lamp easier than a voltmeter for that one. Not the harness, the actual pin.

There's only so many places to look. I'd still hit it with some WD-40 or contact cleaner AND some dielectric grease.

Thanks woodchuck2 & wynot. I have been fighting this thing for months and the old frustration meter finally redlined. I went down to the GM dealer and bought a new flasher. $100.00 well spent, it fixed the problem. I'm a happy camper now, although I long for the days when a flasher cost a couple of bucks.