For service and parts needs, visit your dealer. You will receive genuine GM parts and GM-trained and supported service people.
Genuine GM parts have one of these marks:

Vehicle Care
California Proposition 65 WarningSafety Belt Extender
If the vehicle's safety belt will fasten around you, you should use it.
But if a safety belt is not long enough, your dealer will order you an extender.
When you go in to order it, take the heaviest coat you will wear, so the extender
will be long enough for you. To help avoid personal inju ...
Test Probe Caution
Caution: Do not insert test equipment probes (DMM etc.) into any
connector or fuse block terminal. The diameter of the test probes will
deform most terminals. A deformed terminal will cause a poor connection, which
will result in a system failure. Always use the EL-35616
GM-Approved Terminal T ...
Installation Procedure
Remove rear end upper panel extension (1) from service part.
Cut
Cut the body side inner panel in corresponding locations to fit the
remaining original panel. The sectioning joint should be trimmed to
allow a gap of one-and-one-half-times the metal thickness at the sectioni ...