Electrical: door open trigger

Hi guys,

Just installed an alarm over the weekend (partially) and having trouble locating the ground (black) wire for registering door open/close.

The purpose of course is you want the thing to detect if the doors open (i.e. when the dome light comes on) but couldn't seem to find a wire that wasn't a constant power source.

i.e. no matter which wire I was touching, I'd close the doors, the dome light would go off, but my test light stayed on.

I'll be askin the dealership this weekend if they have any idea (probably not) but hopefully someone has some suggestions.

Thanks,

A.