I looked at getting the harness that attache to the belts in the back seat but then read a lot og negatives about this on the board. I was going to talk to the BMWCCA guys at our next event. I agree I am not going to add any harness bars yet to the back however would like to have a harness because I am moving around to much around hard cornering.
From what I read is if you don't have a roll bar then don't do harness due to no support if you roll over and you are locked in.
The Verdict: Every other manufacturer should give up on building their own cars and just make M3s instead. -Car and Driver June 2008