Check out the 240sx Convertible Facts, FAQs and Stickies thread, it's at the top of the 240sxconvertible page:
240sx-convertible-facts-faqs-and-stickies-t199318.html
Your problem may be the rear window guides. Here's an awsome how-to for fixing them...
zerothread?id=383252
It's also not uncommon for the rear window switches themselves to go bad.
There are also a bunch of other links in the 240sx Convertible Facts, FAQs and Stickies thread that you will probably find useful later on
