in regards to a factory alarm in 91 m5; although you may think it is prewired it in unlikely that a factory alarm will plug in. I recently faced the same quandry and found to my dismay that back in 1991 the alarms used in the m5 were different and required much wiring to install. since then bmw superceded there alarms to one which will not work because the wiring harness beneath the rear seat is not present. I was able to purchase the last known alarm kit (lots of wiring!) from bmw which was located at a dealer in texas. Go aftermarket my friend!! goodluck !