2008-09 Cadillac SRX vehicles with both push-button start and remote start used a pair of 5-button remotes with FCC ID M3N5WY7777A. The original keyfobs came with Fob 1 and Fob 2 markings on them, but the remotes are interchangeable, so it does not matter which you buy. There are multiple GM part numbers for the two remotes. The most common are 20998281 (fob 1) and 20998282 (fob 2). You can view or order the part at the following link:
Cadillac SRX vehicles that use this remote are user programmable if the driver information center or odometer menu on a specific vehicle supports adding a new remote. If not, a scan tool is required for programming.