Check coolant level Check to make sure heater core is not air bound Replace thermostat Check the operation of the heater control valve/cable
I would bet it is a short in the master switch on the drivers door. == Another possiblity== I had that problem. As it were, I was looking up under the dash for a burned out lamp and saw a spark that co-incided with the door lock problem. One spark, one click. The problem was that the wiring harness that contains that particular wire travels behind the parking brake pedal. Over time the harness had rubbed its way through and was shorting out against the brake pedal mechanism. A little electrical tape and a relocation of the harness solved the problem. No more possessed doorlocks.