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