Sensor problems

[Jackmroz]

Living in central Wisconsin during the winters our vehicles are trashed by the salt dumped on the roads to keep them passable.  My truck, '08 GMC 2500HD with the 6.0 gas engine, just turned 175000 miles. About two months ago I got a little glowing red light on the dash.  It turned out it was a sensor for the air bag system.  I have one of the front sensor replaced with a GM sensor, 25820342, which cleared up that problem but I keep getting a warning on my dash that says "Service Trailer Brake System".  My friends tester, a very expensive Sun tester, does not indicate any fault in the trailer wiring but this warning keeps popping up.  Any ideas where I can look to fix this annoying problem? 

[Formulabruce]

I'm not 100% sure of its location in and '08 model, but I would check the ECM Grounds on the frame. On Classic trucks till '07 this is on the frame on a body mount just below the drivers door. on the rear side of the frame mount the body mounts too. The grounding rings on end of wires may have to be replaced. I would solder what ever you do. Then when all done coat with waterproof grease ( get some at a marina) . Good luck !