GMC 2004 Sierra 1500 Pickup Owner's Manual page 394

If your vehicle is equipped with the "Heavy-Duty
Trailering option, please refer to Heavy-Duty Trailer
Wiring Package , earlier in this section.
When the camper-wiring harness is ordered without the
heavy-duty trailering package, an eight-wire harness
with a seven-pin connector and separate CHMSL
connector are located at the rear of the vehicle and are
tied to the vehicle's frame.
4-98
Instrument Panel Jumper Wiring Harness
{
CAUTION:
Be sure to use only the correct trailer brake
harness, the one intended for use on your
vehicle. If you use some other trailer brake
harness, even if it seems to fit, your trailer
brakes may not work at all. You could have a
crash in which you or others could be injured.
Use only the trailer brake harness intended for
your vehicle. If it is no longer available to you,
be sure to get a proper replacement from your
dealer.