My '85 was doing the same thing, but only during the cold winter months. It was particularly annoying when it happened at 3:30 in the morning and I had to go down to the detached garage to find out if some prowler was around (of course slipping and falling on the ice and snow wearing, slippers shorts and a t-shirt).
It turns out that the sensor in the rear hatch that tells the car whether the hatch is open (and turns on the interior lights/sets off the alarm) was dirty and/or loose. I'm not sure why the cold weather made it act up, but it did. Never happened when it was warm out.
I took the cheap way out and just yanked the wire off the sensor which put a stop to the false alarms. The sensor is located behind the hatch tool cover, on the bottom of the lower hatch lock housing. It's a plunger inside a spring that has a wire attached to it. Disabling the sensor will eliminate the ability of the rear hatch to set off the alarm if the hatch is opened, and the interior lights will not go on with only the hatch open, either.
I've since replaced the lower hatch assembly and replaced the sensor with a clean one.....still no alarm problems.
My guess is that you should go around and check all of the plungers (like those inside the door jams) that turn the interior lights on/off. The alarm operates off of the circuit with the plungers.
Assume that your car is randomly having its interior lights turned on (setting off the alarm) and start from there.
Hope that helps.
