Chevrolet Avalanche 2004 Owner's Manual page 365

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 may be
included with your vehicle
as part of the heavy-duty
trailer wiring package.
Connect the adapter with the tab pointing up. Then flip
cap on the vehicle's seven-wire harness will lock
onto the tab and help hold the adapter in place. Plug
the four-way round pin connector onto the adapter.
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