Like I said...don't have a 'tour, and I cannot speak definitively about the locks on them. I was hoping someone else might jump on the thought I tossed out there.
However and with that caveat, if the lock works properly using the actual key, then it suggests to me that the power lock motor crapped out on you. Although my last three vehicles have all been FoMoCo products with power locks, the Mustang is the only one that had lock issues. The power mechanism was held to the door with (IIRC) two pop rivets. There was a rod attached to the actuator that then popped loose that would have to be placed in the new actuator assembly and then pop riveted back into place. The door panel had to come off in order to change it out, and the labor was actually more than the actuator. Even with the newer cars having more of the generic modules controlling a lot of functions, power locks are still most likely using a very similar cheap-a$$ actuator. If I can dig out the manual from the '98 Ranger, I will see what they showed for the actuators and compare with the '90 Mustang manual (don't have either vehicle, but I think I still have the shop manuals around the house somewhere).
If my theory holds true, then the real question comes down to how much the convenience of power locks is worth to you or how mechanically inclined you are when it comes to tearing into the door.
