lookin for some help with car security

I know that this is the car audio forum, but as of recommendation from the COT forum, I figured I would post in here and see if I get any responses.

My sister have a Volvo 960 with keyless entry and supposedly an alarm, both factory. She has a tendency to leave her back windows cracked and last night it bit her in the butt. She got broken into and they stole her radio. Didn't break a window or anything, just popped the locks and took the radio, or so that's my guess. Anyways, she now wants to get an aftermarket alarm. My question is, can you put an aftermarket alarm on a system that already is wired for a factory alarm? I don't know squat about car security, well in terms of wiring and such, so i'm looking for a little help. Any advice is appreciated.

Thanks.

PS - and yes, i told her to just roll her windows up from now on and take the faceplate off

they never listen ......

Yea, you can put in another alarm - jut don't ever arm the factory one