4Doors
I did this on a 1997 Sub. I am assuming GM carries there general wiring diagrams forward from year to year. If that is the case here goes.
Locate the orange wire, circuit #140, that feeds the brake light switch. Easyest place to pick up this wire is at the fuse block. Splice into that wire and attach it to terminal #87, the normally open side, of a standard 30 amp automotive relay.
Loate the white wire, circuit #17, that is the output from the brake light switch to the directional signal. You can find this wire in the harrnass leading to the steering column. You can tell the right side of the wire by appling the brakes and checking with a votlmeter. Splice into the side of the wire that "did not" had voltage on it while the switch was applied and connect it to terminal #30. Splice into the other side of the cut wire and attach it to terminal #87a of the realy, the normaly closed side of the relay.
Locate the output side of your exhaust brake switch and connect a wire from it to one side of the relay coil, #85. Ground the other side of the coil, #86.
Connected this way your brake lights on both the truck and trailer will work normally as if nothing had been changed. Your electric trailer brakes will work. When you actuate the exhaust the relay will open the trailer brake path and apply voltage to the brake light circuit causing the truck and trailer brake light to remain on as long as the exhaust brake is actuated.
If during this time you have to hit your brakes your trailer brakes will work as they are then still feed through the brake light switch.
Locating the output side of the brake switch is the hard part because of the way GM tucks the switch up over the steering column. But it is in the harrness you just have to look and do some testing with a pin and a volt meter to locate the right wire.
If you find your wiring colors are different e mail me and i will send you a schemitic.