The door locks okay, but when the engine is turned off the lock won't unlock without help from the door handle.
Drivers door lock won't open on 1999 Mercury Mountaineer
2 relays located in the rear of the vehicle control your lock/unlock. they're right above the rear wheel well on teh drivers side. due to having the same problem recently, i know that on my 98 the lock wire on 98s is pink w/yellow or orange stripe, and the unlock wire is pink with a green stripe. it should be the same on the 99s also. on my vehicle, the solder point on the wires (under trim on drivers side, right at rear door) was completely corroded inside the heat shrink. i removed the heat shrink, cleaned and re-soldered the wire. worked perfectly. for the lock wire (my 98 wouldn't lock OR unlock) i couldn't find the damaged point in the old wire, so i ran a new wire from a point in the old wire just after the relay where i knew the wire was still good, along the harness and soldered it to the same wire right before the door locks. hope this helps.
possibly your battery is getting weak and needs to have the extra power that the alternator provides
I have same problem with my 1008 Mountaineer