GMC 2010 Sierra 1500 Crew Cab Owner's Manual page 457

The vehicle is equipped with one of the following wiring
harnesses for towing a trailer or hauling a slide-in
camper.
All regular, extended cab and crew cab pickups have a
seven wire trailer towing harness.
For vehicles not equipped with heavy duty trailering, the
harness is secured to the vehicle's frame behind the
spare tire mount. The harness requires the installation
of a trailer connector, which is available through your
dealer.
If towing a light duty trailer with a standard four way
round pin connector, an adapter is available from your
dealer.
For vehicles equipped with heavy duty trailering, the
harness is connected to a bracket on the hitch platform.
The seven-wire harness contains the following trailer
circuits:
Yellow: Left Stop/Turn Signal
Dark Green: Right Stop/Turn Signal
Brown: Taillamps
White: Ground
Light Green: Back-up Lamps
Red: Battery Feed*
Dark Blue: Trailer Brake*
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