electric door locks

I have a 2003 GMC Safari and the door locks do not work with either key fob or inside controls. Fuse appears ok. All other power acessories work. When the controls are pressed you can hear clicking in the doors like the actuators are getting power, but not enough. They quit in a sequential manner over a couple months time- First the drivers door, then the passenger door, then the rear , and finally the side door. I'm the original owner and nothing has ever been rewired or added. I have a 1993 wiring diagram that shows a door lock relay and a module as two separate items, but don't know where located. It may be the ground connection, but don't know where that occurs. Can anyone help?