When you say motors, I assume you're referring to the door lock actuators located inside the door and mounted up against the jamb. The aftermarket actuators tend to get moisture up under the boot on top and freeze up the actuators. Your best bet is to get new Ford actuators except they're discontinued and if you find some you could probably buy a new Mustang for the price the seller would want!
Buy another set of aftermarket actuators, make sure your plastic door moisture barrier is intact, seal up the door real good and even throw a couple of dessicant bags inside for good measure.
Just simply a bad design from Ford.
__________________ '89 LX ragtop with turbo'd TC motor - Thunderstang!
'86 Turbo Coupe (daily driver)
'88 Turbo Coupe (parts car) | 
why no door locks! 
