[rotor_wash]

Pat, I believe the correct documentation is GM Document ID #1706194. You will have to get it from a dealer. They should be knowledgeable on the subject because of all the complaints about folks buying a trailering option only to find it is no longer hooked up. Only the 30 and 40 amp fuses are installed. They now blunt the wires below the dash for the plug to a trailer brake (such as your prodigy controller) requiring you to install the plug. No more "plug and play".

From the manual: The red/black circuit 242 (trailer brake)must be connected to Stud 1 (30 amp) of the UBEC (fuze box). The wire is located between the left fender and the UBEC. Do not confuse this circuit with 742 (document #1706194) which is also red/black, and which is taped to the harness under the master cylinder. Circuit 742 is the 12 V supply to the 7 way trailer connector.

(thank goodness for pictures!)

40 Amp fused stud for aux power, Fuse 71 = Stud 1 (40A) circuit 742, size 8 nut
30 amp fused stud for brakes, Fuse 66 = Stud 2 (30A), circuit 242,size 5 nut
Both wires are red/black.

First, I would check the 12V #4 position on the Bargman circuit for power, then I would check that a fuze is installed (40) amp, and lastly that the circuit is actually connected.

Of course, with your 99 Sierra, your mileage may vary...