I have a 1999 Lincoln continental and the door ajar light keeps coming on also the gear data display error

Had the same problem the rear door latches need to be lubricated when not used they tend to send a code to the computer telling it there is a door ajar.

Before spending +100 for a mechanic try spraying the latch with wd-40 or some lime off followed by olive oil once the lime off dries out. A lot of times the latch starts sticking so removing all of the dirt and grime along with an oil or lubricant will clean and loosen it back up to working condition.