From all I've read, I've decided that I would like a rear sway bar. Someone has a new factory rear sway bar from a 2005 MCS that has not been used. It includes the OEM bushings and brackets. Can anyone tell me if it would fit on a 2003 Cooper? If it would fit my R50, then I'd like to buy it, but I don't know about these kinds of things.

 

From all I read and remember from doing mine (my 03 Cooper and then my 06 S), yes it will fit any Cooper or S 03-06.
I would suggest that going through all the work of changing it you should go for a heavier (after market) bar. They usually run under $200. I installed the Mini Madness 22mm bar and love it!!! If your deal falls through and you still want just the S bar I have the one I removed with maybe 1000 miles on it.
Blake

 

The stock rear bar will probably work better for you than a larger aftermarket bar if you don't currently have the sport suspension.

If you would like to add a sport suspension I have one with 28,000 miles on it (installed a JCW suspension in its place). PM me if interested.

 

I had the sport plus rear sway bar (S rear sway bar) replaced on my MC. I went with the H-Sport 19 mm, $288 installed made a world of difference.