GMC 2004 Sierra 1500 Pickup Owner's Manual page 395

This harness is included with your vehicle as part of the
heavy-duty trailer wiring package.
This harness is for an electric brake controller and
includes a trailer battery feed fuse. It should be installed
by your dealer or a qualified service center.
Four-Wire Harness Adapter
Use this adapter to connect a standard four-way round
pin connector to the seven-wire harness on your
vehicle.
This adapter is included
with your vehicle as part of
the heavy-duty trailer
wiring package.
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