I have the same problem on my 2007 roadster, a narrow wear mark about 3" long 12" forward from the top of the rear window and almost dead center from the sides of the car. I think it's caused from the locking mechanism on the underneath side of the rear deck that covers the top after it lowered. I'm going to put white chalk on the locking hook and see if it shows up on my top when I put it up. If that's it, i'll file and polish the edges of the hook and hope that takes care of the problem.