WARNING: This page is about a different car, the 2003 Chevrolet Monte Carlo and 2003 Chevrolet Impala. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Rearview Mirror & Lamp Components
Courtesy of GENERAL MOTORS CORP.
Install the bulb to the socket.
Align the lamp lens to the rearview mirror. Press the lens into place.