Hi,
Check out this alternative to replacing your Top if the Rear Window Cracks. It a pretty cost effective alternative to spending $1500 for a new Top when all you need is the window. Check out the link:
http://cgi.ebay.com/ebaymotors/Porsche-Boxster-Plastic-Window-911-Convertible_W0QQcmdZViewItemQQcategoryZ33652QQitem Z8057928681QQrdZ1
Hope this helps...
Happy Motoring!... Jim'99