Chevrolet 2005 Silverado 1500 Pickup Owner's Manual page 388

Four-Wire Harness Adapter
Use this adapter to connect a standard four-way
round pin connector to the seven-wire harness on
your vehicle.
4-98
This adapter is included
with your vehicle as part of
the heavy-duty trailer
wiring package.
Connect the adapter with the tab pointing up (see
arrow). The 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.