98' Mustang Car Alarm

Sounds like its an aftermarket alarm and the brain is probally going bad. Get in the d/s kick panel and under the column and just take it out. If it has an ignition kill swithc you will likely have to reconnect that.

Just look for anything that is not factory. Butt splice connectors, t taps- that sort of thing. Wouldn't hurt to unplug the battery before you do this as well.

i have the same problem with the factory alarm. the alarm will go off for no reason on my 98gt.

I been reading that the door pins and trunk sensor will cause the it to go off.

i have not had the time to disconnect some of the leads to the factory alarm module from sensor to trick the module on not arming but still be able to use keyless remote . i was told module is in the truck area left side.

check the door pins/ domelight wire. check for either a green or purple wire on the alarm. that wire is usually the door "trigger" for most aftermarket alarms. connection should be made in driver kick panel.
i dont belive the 98 has a gem module? does it? if so the alarm has to be isolated with diodes to prevent back feeding to the aftermarket alarm.

The factory alarm is a joke anyways.

here is a reponse i got from a ford tech on disabling the factory alarm.

"looking into your problem and if that trunk tamper pin or open/close sensor is messed up that will cause your problem. After looking at the wiring for the alarm, the module needs to see inputs from both front doors and the trunk after it is armed. If one is messed with or the ground isn't there for one of those it will go off. You can also disable your factory alarm and still use the unlock/lock on your fob. The module is in the trunk on the left side and it has a natural color connector. If you look at the connector with 6 pins on top and 7 pins on bottom, you want to take away the second and third pin on the bottom counting from the left. The second pin is actually #6 and is for the doors, and the third pin is actually #5 and is for the trunk. All those other pins are for your lights, horn, and all the other stuff that goes off when the alarm ringing. You could probaly just unplug it too, I don't see that causing a problem either. Hope this helps. "

I just unplugged mine and so far so good. The anti theft light is no longer present on the gauge cluster and it sat for a while with no alarm issues. Mine would go off after thirty seconds without fail.