I didn't see anything in my 1998 MB warranty book about extended warranty on safety equipment, just on emission stuff. Maybe the fed DOT or NHTSA web sites will have info about required warranty periods. Or MB may 'GoodWill' the replacement.
OTOH, when our 1988 Acura Legend rec'd notice from the feds that the seatbelts were defective, they initially refused a recall or free repairs, and treatened to fight the Gov in court. But media reported a Japanese Acura VP as saying something about Americans being messy and eating in their cars, gumming up the mechanism. The resulting PR
headach caused Acura to agree to replace entire seat belt assemblies at no charge.
I recently replaced the seat belt on my 1987 300E as a preventative measure. The entire belt, retractor, and explosive charge tensioner are one unit, and was a couple hundred dollars as I recall. No idea what it is for a 1998 S class - maybe Phil in Fastlane can get you the numbers. It didn't look to be a difficult DIY job from watching my tech work. The most difficult part was SAFELY (as in firearms safety!) setting off the old belts powder charge so that it could be disposed of properly and legally.