I haven't checked the parts situation with Ford, but I recently found out that individual motors aren't available from the dealer for a 1998 Honda Accord driver's seat, just the entire seat pan assembly with all motors, for about $1000. In that case the only options are to repair the motor(s) or find a working seat assembly at a salvage yard that can be reupholstered. Considering how little the seat motors run over the life of a car, the motor commutators and brushes are not likely to ever wear out, and the windings aren't likely to burn up unless the motor was powered up and left in a full-power stalled condition for an extended time.
Incidentally, while attempting to service a balky seat back adjuster motor on said Honda Accord, I found that disassembling the motor in place wasn't all that hard to do, giving access to the rotor, bearings and brushes. As long as the gears and linkages are in good order, you may be able to repair a motor in place by borrowing parts from a similar salvaged assembly, without having to completely remove the motor from the seat pan. Honda uses a positive temperature coefficient (PTC), fast acting self-resetting fuse-like device, called a PolySwitch, inside each motor to limit its travel and provide overload protection. If Ford uses the same principle in their seat motors, make sure the PolySwitch is in good condition. It should read near zero (0) ohms when cold; if it reads more than a half ohm, or so, it's probably bad and should be replaced. You can either compare a known-good motor resistance to a malfunctioning one, or measure the PolySwitch resistance once the motor is opened up. The rotor windings, commutator and brushes in a motor may appear to be in perfect working order, yet the motor won't run if the PolySwitch is defective. When they fail, they tend to have high resistance and let too little current through to permit the motor to turn. PolySwitch devices have been around since the early 1980's, so your 1994 Lincoln Mark VIII could be using them, too.