My passenger power door lock doesn't seem to operate either from the driver door or the passenger. Any ideas on what to start checking? Truck is a 1990 F250 Ext. Cab.

 

If it isnt working from either one of the actuator buttons,and the buttons are activating the other side.Then your problem is in the actuator solonoid,( probably just a loose connection),the solonoids dont go out that often,its just an electro magnet,but the connection can work loose from opening and closing the door over time.
Jeff

 

I had this same problem on my '90 F150. My passenger side solenoid quit working. When I removed it I found the rubber boot had torn and water got in a "mucked" up the solenoid. I tried cleaning it out but couldn't get it to work. I bit the bullet and bought a new one from Ford. It was about $50 bucks I believe.
My .02
Herby

 

Actually the "solonoid" is a motor that drives
a captive screw that operates the door lock. Its
not one of Ford's better ideas, because the motor
normally runs a very short time, then stalls as
the screw bottoms out or tops out. As long as
you don't hold the button down, the motor won't
burn up (too much).

But, the motor is protected from the elements by
a rubber boot that just isn't good enough in the
quality dept. When the boot deteriorates, the
rain gets into the motor and kills it.

I've been through two motors in my F250's driver
side door. Last time I decided to cut my losses
and put in a motor from a salvage yard that
specializes in Ford trucks.