I have some good pictures of what I went through fixing my Monte. With the vinyl top on, you could not see a problem. The vinyl top was in great shape - it appeared a previous owner tried to cover up the problem instead of fixing it.
But it appeared that the interior below the rear window had a water problem in the past - that was the only visual evidence of a problem. Once the vinyl was off, and the window was out, I found some extensive rust damage. It's been documented here - take a look.
http://www.katuszonek.com/window.html