I have a 1987, 300 SDL that has an alarm problem when it rains enough to splash the bottom of the car. After I've driven through the rain, parked and locked, the alarm system will not go off, when I unlock the car the next day. Nothing I do will turn it off. It last happened at Christmas when the rain was followed by cold air, freezing everything. The alarm would not go off, when I unlocked the car, for a week. Presumably, the car dried out over that week and then the alarm began to function normally.

The problem doesn't seem to happen in light rains, only when it's raining heavily enough to splash up under the car with some force. I would guess there's a short somewhere under the car. Where should I start looking for it and how would I test for it?

I would appreciate all the help I can get, as soon as I can get it. I have to drive a borrowed thirteen year old rusty, Saturn, full of junk, until I fix it, or it dries out.