If you've central locking already you can get an aftermarket alarm and tap into it. I got one for my E36 solely for the purpose of remote central locking. ebay jobby will do the trick, think the one I got was 40 bucks and worked perfectly. The alarm will send +12V/0V via a relay off you go. Think the alarm I got was a hawk one. You'll have a fob rather than the key and fob being all the one.