It has to be either a short in one of the door jam switches or a short somewhere in one of the wires. I would start with visually inspecting all four of the switches, I dont think it would be the back lift gate because you said that the courtesy lights stay on, not the cargo lamp. Maybe you accidentally closed the door with something in the way and broke or pinched a wire. This happened on my friends BMW 750...he didnt know it was the door switch because he always left his courtesy lights off. We noticed the nipple of the door jam switch had broken off and was not being pushed in....this tricked the alarm into thinking the door was being opened, and the alarm would go off.

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1994 S320
131K miles

1998 ML 320
126K miles

1994 Blue C220
122K miles

Im going to Grad School @ the University of Iowa
Go Hawkeyes!!!