As a prospective new owner I would be very grateful for some help. I have read much of the helpful info on this forum but have a question to which I can't find the answer. I am looking at a 2003 Spyder which must be one of the last with a plastic rear window. However the window has a zip round the outside & a Velcro flap at the bottom. Can anyone tell me if this makes the window easily changeable by just zipping the old out & the new in or does this zip have some other function?