The passenger seatbelt in my '02 Cooper S stopped retracting allowing no passengers (safely or legally) to ride along. I ordered a new replacement and found it did not fit - maybe for an R50. While I had the original one out I blew the mechanism out with compressed air and added drops of 3 in 1 oil at every opening I could find in the casing. After extending and retracting the belt several times the belt tensioner started working again with plent of power. So, try this before you order a replacement. FYI - Getting the original mechanism out was surprisingly easy - 2 bolts and 1 small screw.

I hope this helps someone.

Joe Mc