Dude, i just went thru a similar nightmare. And a nightmare it was

Secondly, If it came with the car, dont mean nothting. It could have been installed at anytime sense the car has seen the street. (and remember that basic alarms are only like $20 at autozone) so it's the more *popular* option
Start by getting under your stearing colom, Remove the 2 philips screws holding the panel with your dimmer switch and key hole and remove that panel. Then remove the panel above your knees in driving position. 4 philips and 2 "clips". Undo the hood release; 2 philips. Then you have to remove the bracket behind that pannel. 4 10MM bolts. Now you should be able to find a motion sensor, (box with a red and green light *usualy*) This is the most common place to mount thees things.
Now follow the wiring band up to the "brain" of the unit. No telling where that's stuffed. You'll just have to follow the wiring. When you get to that 'brain' then you simply need to discounect EVERYTHING that it's attached to. MAKE SURE THAT IF SOMEONE HAS SPLICED THE WIRING THEN YOU PUT IT ALL BACK PROPERLY!! otherwise your car may not start, windows may not operate, doorlocks may not work, ect.
G/L