but its always good to have a guide before testing
also find some alarm help at my homepage:
http://audio.morningz.com
Also for the record: for the most part Fords and a few Mercedes have 2 wire (+)-pulse based doorlock systems, the rest of the universe uses (-) trigger
also Kerm, check out the electrical-based articles in the Articles section (big link on the left of the screen), lots of things you have wondered about are already there, and with pretty pictures