2003 Mercury sable door ajar and dome light stays on

Spray some WD40 into the door latch worked when I had that problem.

Use a lot of the WD-40, then operate the doors and the latch mechanisms for a few minutes each and open and close the doors as well to get it into every parts of them. This is a known moisture infiltration & corrosion problem with these lock/switch combinations. There are also 2 small switch solenoid type attachments on the latch mechanisms, and it is possible that one or more could have failed if the WD-40 thing does not work for you. When my son's went bad, it would also trigger the alarm intermittently but frequently & the WD-40 worked great!