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Yep I figured that out too! Here's another pic for location...But basically follow the fuel cell toward the front of the truck until the first subframe and its attached to it.
View attachment 228899
Update....even thou I changed the relay and multiple fuses....I am still getting the warning WTF!
It happens randomly, but also seems to happen when braking hard downhill...So I still think it could be a ground wire someplace.
What would be nice if the great engineers at GMC would have programed shortcuts in the all control modules to divert or supercede non critical warning..instead of Go/No-GO...feels like Windows 98 level programing
Welcome to the forum Bruce. Must have been a last minute change as the manuals show everywhere else for that thing!This is a pretty old post, but I was looking for the same relay on my 09 Tahoe. I found tons of videos of where to find it on a Silverado, but nothing for a Tahoe or Yukon. I found it on the driver side of the crossmember in front of the fuel tank. Its impossible to get a torx wrench on the back side, but you can use a 4mm socket and loosen the bolts from the front side. If you loosen them a little bit, then you can slide the bolts in the slot and then the relay will come loose. And as many before me have said, replacing the relay got rid of that pesky "Service Trailer Brake System" light! If anyone is looking, I hope this helps.