FYI Bob, I have a '97 960 Wagon with the guide on the driver's side missing a few small pieces of plastic. This makes it "clunk" a bit when closing from the vent position. It slides back fine. I asked about removing the remaining plastic a few weeks back and got an email from a Volvo mechanic saying he had see this done without any apparent ill-effects. For the time being, I'm leaving it alone and will not use the vent function. I also have the problem with it leaking between the glass and the metal frame and will address this when the weather get a little more reasonable around here (MA).

When/if you decide to replace the guides, there is a Volvo service bulletin which outlines how to do it with illustrations. You can get it from the Volvo site buy purchasing a 3-day subscription. This gives you access to every service bulletin on every car and was the best $3.00 I ever spent.