[truckster]

It was originally set with glass setting tape, which is a compressible rubber tape that contains fibers in it. A glass shop that does classic vehicles should be able to help you with that.

Newer vehicles have the glass set with epoxy. I like JB Quick Weld, because it sets fast and it has a similar flexibility to the professional epoxy. If you're going to do it this way, you want to make sure you have the track in the proper position.

Clean any loose rust off the track, and clean the bottom of the glass well. Make a couple of shims from thin cardboard. Make a U out of the cardboard shims and put one on the front and one of the back of the track. Set the glass in the track and run it up and down to make sure it's not going to bind. Use a piece of tape to mark the position of the track on the glass, and then mix up your epoxy. You don't need to fill the entire track with epoxy, a couple of inches at each end will do. Use the cardboard as a spacer to center the glass in the track and glue it in. Let it set up for a few minutes and you're done.