GMC Sierra Classic 2007 Owner's Manual page 452

Maintenance When Trailer Towing
Your vehicle will need service more often when
you're pulling a trailer. See Scheduled
Maintenance (Gasoline Engine) on page 613 for
more information. Things that are especially
important in trailer operation are automatic
transmission fluid (don't overfill), engine oil, axle
lubricant, belt, cooling system and brake system.
Each of these is covered in this manual, and
the Index will help you find them quickly. If you're
trailering, it's a good idea to review these
sections before you start your trip.
Check periodically to see that all hitch nuts and
bolts are tight.
Trailer Wiring Harness
Your vehicle is equipped with one of the following
wiring harnesses for towing a trailer or hauling
a slide-in camper.
Basic Trailer Wiring
All regular, extended cab and crew cab pickups
have an eight-wire trailer towing harness.
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For vehicles not equipped with heavy duty
trailering, the harness is clipped 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 your trailer is equipped with electric brakes, you
can get a jumper harness (electric trailer brake
control) with a trailer battery feed fuse from
your dealer.
If you need to tow a light-duty trailer with a
standard four-way round pin connector, you can
also get an adapter from your dealer.