try using a window cleaner. Im not sure what its called but its made for plastic convertable windows and is supposed to return them to their clear state. If the line is still visible, Im not really sure how to take it out, but I remember when my dad had a z3 he used to put a towel on the back window to stop the crease from forming. Not too practical if you're going to be up and down a lot though.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>