Typically the cause of a window not rolling up or rolling up slowly is a bad motor, especially if the car is old and the motor has never been replaced.
However, there are several other possibilities such as faulty wring, blown fuse, or a bad switch.
Faulty wiring is easily checked with an ohm meter.
A blown fuse is also easily checked by checking the fuse controlling the windows on your fuse panel.
Another possibility, which is often the case in 90's Ford cars (though perhaps not for the 86 T-Bird), is the passenger side window switch is dead. If it is then the window will not roll down AND it will not roll down if you are using a switch for that window on the drivers side (if your car is equipped with two switches for the passenger window). On the 85 T-Bird both switches are on the same connection. If this is the case for the 86, then this is not the problem. However, you might want to check the switch anyway, because taking the door panel off for the 80's t-birds to get at the window motor is a pain.
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