WARNING: This page is about a different car, the 2007 GMC Sierra and 2007 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Pressure Relief Valve
Courtesy of GENERAL MOTORS CORP.
Align the pressure relief valve into the opening of the rear cab.
Press the Pressure Relief Valve into the Cab Back Panel until the locking tabs are fully seated.