If the aftermarket alarm still works with fuse#15 pulled and pulling 15 kills the lights. I would say that the aftermarket alarm is not directly the problem.

The factory alarm is under the passengers floorboard. Pull the carpets and remove the wooden or plastic floor panel. The alarm unit is to the center, it is black, it should have two connectors on it, and probably will say "Becker" on it. Try unplugging it. If that still has no effect, try to find the pump for the door locks and unplug it. It should be in the sparetire well encased in black foam. It could be under the rear seat, I just don't remember.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician