1998 Mercury Villager automatic door locks

Electrical problem
1998 Mercury Villager Two Wheel Drive Automatic 73000 miles

I have a problem with all the door locks. When I attempt to lock the doors by using the inside lock button, the "switch" grinds "and often does not lock the doors--the same is true when I attempt to unlock. Also, all doors will sometimes lock"automatically" when I shut the drivers side door after the engine is started. And, when I attempt to unlock the doors, sometimes only one, two, etc. Will lock or unlock.I never know what each door will do when I attempt to lock or unlock. Switches, motors? Thanks.