99 Mercedes ML 320 Seat Belt Failure
 

Drivers side seat belt now failing to retract. It does not seem to have the retraction pull that it should have from the floor unit, which appears to be "lifeless". Can The floor unit be rewound to recreate the tension action or must it be replaced, either totally -- or with just the winding unit replaced.

Is it better to just buy a whole new seat belt system including the floor unit ??
OR
Is it possible to just replace the floor unit winding spool only?