Old Man Dirt
Favorite Answer
This is about as much work as replacing the top and presents special problems. The plastic rear windows change according to the maker if it is a zip up rear window. If you have a window that is made into the top, it is even more of a problem.
The top has to come apart enough to separate the window from the top. The back carpet has to be pulled out and then a retaining rail removed. If the top is original (and some replacement ones also are installed this way) rivets have to be drilled out. Then the window can be separated from the top and the new one installed.
The difference in prices between a complete top and just the window are not different enough to justify just doing the window (IMHO). The labor price is almost the same also.
When my windows go bad, I just replace the top and take better care of the next one.
I use the Keen Auto three window top by the way. It looks like the old MG and Triumph tops and helps me feel less closed in. Go Miata sells them, but the picture is not right on the site.
http://www.gomiata.com/keeautotop.html
You can find directions on this at Miata.net (http://www.miata.net/garage/garagetops.html)