I assume you first checked the fuse for that circuit. If there was a
short the fuse would be blown. Next push the driver's door lock button
down and listen for a clicking sound coming from under the driver's side
dash panel near the fuse box. This is where the door lock relay module
is located. If you don't hear the click, you can check the driver's door
lock switch without removing the door panel, by accessing the wiring to
that switch at the door lock relay module. It's located on a bracket
along with other modules, above and behind the interior fuse box. Remove
the wiring connector from the module. Use a meter to test for continuity
in the door switch wiring at the connector. The resistance should go
from an open circuit when the button is up, to very low in the down
position. You may need a wiring diagram to figure out which wires from
the module connector go to the driver's door lock switch.
In my case the driver's door lock switch wasn't the problem. I could
hear the relay module clicking when I pushed the driver's door lock
button down, which meant the door switch was working, but the other door
locks didn't go down. I discovered that I could unlock the doors, if I
first pushed the other three door buttons down by hand, before lifting
the driver's door lock button up. This is because there are two relays
in the module, one for locking and the other for unlocking. I did some
micro surgery to remove the cover of the locking relay and found it had
worn contacts. I resurfaced them with fine sandpaper followed by a light
application of electrical cleaner. I wouldn't advise trying that unless
you've had some experience with relays. Otherwise get another door lock
relay module from a junkyard for about $30 or less. A new one from Ford
is outrageous, like $100!