there are plastic clips that hold the window to the track. They're very hard to come by, but if you check
http://www.car-part.com, you can see if any yards have them in stock (this is what I did when I repaired an SR vert I was getting ready to sell). Another source to try are the people at Courtesy Nissan. More than likely, you'll have to order the plate assembly that the clips are attached to. Most dealerships seem to think you're talking about the window regulator when you start talking about the quarter windows. Make sure they understand that you're talking about the plate that connects to the window itself and sits in the rails.