I'm looking at the same issue. I found this series of comments and am about to try dismantling mine to see what's up with it:
http://groups.msn.com/autorepaironline/fordproblem.msnw?action=get_message&mview=0&ID_Message=18180&LastModified=4675448067745187088
Not really a hard job to do ,would say go to your local junk yard and see if you can find one .pop off trim panels ,they just snap off and on ,and unscrew the whole setup motor and all,there is a elc plug at both ends to unplug and its yours ,eazy job Slim
A Google search turned up your post, the above, and a general pic and description of repair:
http://www.i-car.com/pdf/upcr/procedures/re/re11.pdf
and one more advice location: http://www.autosite.com/garage/subsys/barestsy.asp
The drive motor is usually located at the base of the track assembly behind the rear seat side trim panel. Its purpose is to pull the tape that positions the belt. If a check of the drive motor reveals that it is faulty, replace it. Like any motorized system, the seat belt parts need periodic lubrication.
To service the motorized seat belt system, follow the instruction given in the service manual.
Wyatt
July 2004