Next step was to scour the net looking for a stand alone system but after weeks of searching I could not find what I wanted. I really just needed something that could be left in the cold, had a motion sensor and an alarm/siren and did not operate on batteries. No one makes a basic system like that that I could find so I decided to build my own.
Went to home depot and bought the following items:
outdoor motion sensor with 2 light sockets - $14.95
2 bulbs for sensor - $8
16v transformer - $10
knockout electrical box - $1
Then I stopped over at radio shack and bought a 108db 12v piezo siren for $9.
I wired everything up and mounted it on a piece of scrap mdf. I had an old shopvac that has a blown motor so I cut the cord off and wired it into the system for power. I spliced the transformer into the lines for the light sockets so that the alarm only sounds when the lights go on (motion is detected).
This gave me the basic system that I wanted but the problem was arming and disarming the system. I did not want it to go off every time I opened the door. The solution was found at Target for $10. I bought a westinghouse remote control electircal thingy. It has one piece that plugs into an outlet and then has 2 outlets built into it. It comes with a wireless remote and it's intended purpose is to allow you to turn christmas lights on and off without having to unplug them. Works like a charm for my needs.
Now I have a motion sensor alarm that is loud as hell (12v siren getting 16v), 2 flood lights, it can be turned on and off from the car, no batteries to worry about dying in the cold and only goes off if someone enters the garage from either door. Total cost was around $60 after tax for everything.
Funny enough, I was just dreaming up a setup like this in class today. 
