I would strongly advice against having any aftermarket equipment installed by the teenage morons who are usually employed to do these things. Oftentimes you will end up with mysterious electrical problems that are not immediately apparent.
Volvo has a factory alarm either as standard or as an accessory that fits into existing sockets and wiring if I am not mistaken. This is far preferable to the cut and splice methods used by aftermarket installers. Keep it simple and factory std. to avoid problems. I speak from experience trying to fix numerous wiring problems caused by installers.