My '76 DeVille has the factory alarm system in it and tonight after being armed for a few hours just went off by itself. I live in a rural area so I really doubt someone touched it or tried to break into it so I'm confused. What do you have to do to set the alarm off in that car?

Is it humid where condensation can ground out the circuit ? I have a 96 (yes, I know, different animal) What I found was the door lock cylinder retainer was corroded and had some build up. When it rained or was humid out, at night (2am - 3 am) the condensation would cause the alarm to go off intermittently. If I slamed the door, things would be ok for a while. I don't know if you have the wiring diagram, but it may help to look and see what parts might be subject to humid/damp conditions and may be affected by movement (like opening/closing or locking/unlocking the door) This is where I would start.

Good idea. My DeVille is actually missing the rupper lip that seals the window on the outside from the bottom so anything can fall down that half inch space. Would that do it?

If I remember correctly this system has a switch on the door locks and trunk lock plus a switch under the hood. It detects tampering with door or trunk locks and if the hood is forced open. I would check the hood switch first as it's easiest followed by trunk then doors. Hope this helps.
Alan.

It went off again last night, this time it was armed for a good 8 hours. Where can I find these switches I should look at and what should I look for? Thanks for any help I can get. I figure if I've got the super rare factory alarm, I'd like for it to work properly....

Hi,
The hood switch is mounted on the left hand side of the radiator cradle tie bar, its the same as the interior light switch mounted in the door pillars. The lock switches are part of the locks themselves. I believe each one can be unplugged and isolated to test the system. I suppose it's possible the control unit itself is causing the fault, but this would have to be tested by substitution.
Alan