van360

I had a '95 Sebring with the same issue. My aftermarket alarm guy told me it was supposed to be 5 volts on a certain wire to kill the factory alarm. He told me to use a certain resistor chip to reduce the voltage with a jumper cable on negative. It blew a fuse I replaced it,but it worked. If you take a good power probe to a fuse box you will notice not everything is 12 volts positive or negative, some circuits are in between. Chrysler product alarm circuits for example.
 
When good technicians install aftermarket alarms they are supposed to be integrated into the factory alarm system to make sure they're both shut down. 
 
A couple times I programmed a Ford remote and it shut down the aftermarket alarm because they're hooked up to each other. 
 
I have a client that lives in a bad part of town that owns a 2003 acura MDX. She has a factory remote, criminals kept using a jiggle key on the door lock and because the electric switch was hooked up to it it would shut down her alarm.  Several times she had her ignition turned on but didn't steal it.
I referred her to my aftermarket specialist who made sure to install a diode on the integration circuit. The aftermarket alarm still needs to shut down the factory alarm, but turning a jiggle key in the door lock and shutting down the factory alarm will not shut down the aftermarket one.