I just fixed my driver's door LOCK switch connection, it's the ground wire that goes to the LOCK button, (1997 Aerostar); There is a great writeup on troubleshooting this problem in the Aerostar Forum by BrooklynBay. The driver's UNLOCK button worked fine but the LOCK button did not.
to test this you need to dismantle and remove the door upholstery panel and the kick panel by the left foot.
follow the harness that crosses over from the door through the door jamb, (through the rubber accordion boot); follow the wires to find the (usually grey) connector that connects to the main harness inside the vehicle;
now look at the rear of the LOCK switch button and find the similar color wire at the rear of the LOCK button contact;
get a new more or less similar gage wire and connect one end of it to the grey connector wire that corresponds to the LOCK button wire, then make contact with the other end of the new wire to the back of the LOCK switch while pressing the LOCK button. (three hands are helpful here, but I'm not calling anyone a freak).
Chances are (like Johnny Mathis sings), that the LOCK button switch will now work assuming the new wire is making good contact at both points. You found el problema.
to make a permanent repair, get a pair of plastic wire clamps/splicers and splice the old wire with the new wire to the grey connector wire then route the new wire through the rubber accordion boot into the door, (use a clothes hanger if needed), and once inside the door, start to spiral the new wire around the door harness until you get to the back of the switch, the original wire is severed somewhere between both points, and not worth wasting the time trying to find where it is severed. to hell with it, I've had two Aerostar with the same problem. Probably due to the fact that the driver's door gets opened 10 times as much as the passenger door.
if you want to do a professional job, get some vinyl ties or electrical tape and strap/tape the new wire about every 3 inches around the original harness to prevent any conflict with the window mechanism or what have you.
good luck; if you do it right you only do it once.