I had an ongoing problem with the Auto on the Fairmont kicking into limp home mode (Stuck in 3rd) for no apparent reason, particularly when it was wet. This had become quite annoying. The CarDoor open lights seemed to go haywire as well, flashing the doors open and closed.
Well, a couple of weekends ago, I was giving the car a clean and decided to look at the interior light switches, as the drivers door was rather erratic with the light coming on.
Pulled the switch out, and the contacts at the back were covered in corrosion. Ended up giving them all a clean with the bosses nail file (No emery paper handy - got in trouble with her but that's life!). Lights come on as expected etc. Have also noticed that the problem with the gearbox is no more. Have no idea why this worked, and whether the aftermarket alarm had anything to do with it, but I am now very happy about it.
Recommend that anyone having weird problems with the auto, try cleaning the door switches for the interior lights.