Before you cough up a big chunk of cash, check out Crutchfield -
http://[url]http://www.crutchfield.com[/url]
They have custom wiring harnesses that they sell with their alarms that plug right into the factory electrical system. I wish I'd known about this when I put my first alarm in my GMC S15 several years ago. Tracing down all the wires for stuff like door locks, starter interrupt, dome lights, parking lights, ignition, etc., and splicing into the circuits was a huge pain!!! I spent many hours on my back on the floor of the truck trying to get it all right.
But now that my Excalibur is starting to flake out, I bought a new alarm system from Crutchfield and it's all gonna just plug into the GM harness. The bad part is I have to remove the old alarm and return the wiring back to original condition and undo all the damage I did the first time around.
So instead of paying some shop several hundred bucks to let some $6/hr. flunky go to town with a pair of wire cutters under your dash, consider plugging it in yourself and getting exactly what you want.