Speaking of alarms ...
Ministry of Bass have a good selection of car alarms that they sell and install. Just upgraded my car security with them. They're more than happy to layby your goods. So if you want to get a bunch of audio gear and car alarm, you can pay it off and once you've paid enough for the alarm price, they'll happily fit the alarm while the rest of your audio install continues on layby.
Had a good chat with them on the weekend. A few horror stories about guys spending $4K on audio gear, and $0 for an alarm. And guess what? The car gets stolen. Alarms are cheap, protect yourself.