I agree with Sharon.

I had the exact same problem with my '98 Taurus.  I had the additional
symptom of the horn double-honking when the doors were locked by keyless
entry remote (indicating door adjar).

I tried the WD-40 first, but that didn't help much.  I finally broke down
and took the door apart.  I wasn't able to get to the switch easily, but
just removed and replaced the wiring harness and it worked!  That was around
two years ago and still no problem (knock on wood ...).

Good luck!
Pete

I have a 96 Taurus that has a Door Ajar warning and the interior lights stay
on.  By the chime that sounds when I open the other doors, I narrowed it
down to the passenger front door, but I can't find the switch that detects
whether it is open or closed.  I looked in my manual but couldn't find it
there either.  Can anyone tell me where the switch is and how to check or
adjust it?

Thanks.
Steve