My tensioner has started to go on my driver's seat belt and of course I can't just buy the tensioner; I have to buy a complete Safety Belt Retractor. The dealer is quoting me $500 for the part alone so I'm hoping someone here will know of a better place to purchase one.

Try https://www.getmercedesparts.com/oem...des-parts.html.

Part number for a 03-05 Right unit is 23086004859C94 and the left unit's stk # is 239860019C94.

Should be priced at $351.00.....

Good luck.

Awesome. Exactly what I was looking for. Does anyone know how difficult of an install it is?

Try buying yourself a Shop Manual, see
http://www.startekinfo.com/StarTek/outside/9124/?requestedDocId=9124, part number P-2700-230-07.