Door window hassles

Hi Geoff,
Taking the glass out altogether makes it an easier repair but can still be done in the door.
You'll need to make sure the bottom metal "clamp"is in good condition because if it's got a bad case of rust, the same thing will happen again. It relies on spring clamping pressure of that metal section combined with the rubber insert to grip the glass.
You'll need a new rubber insert for the job and that can be in the form of thin rubber strapping which you can buy at a rubber shop. Anything from about 25mm wide is good as you can trim any excess. Doesn't need to have the catching flap like on the original. That's meant to keep water of the mechanisms but rarely works.
Make sure the the bottom of the glass and the inside of the metal clamp bit is clean. I've ocassionally crushed the metall bit a little if the opening has stretch apart. Fit the rubber strap into the metal clamp and you can assemble the glass using a couple of methods but I prefer to try this one first - Squirt or spray the bottom of the glass & rubber with a strong dishwashing liquid solution and force the slippery glass into place. Once this dries off the rubber usually exhibits a good grip and jobs done. If it does let go again due to age & the metal losing its spring, you can substitute the DW liquid for wet contact adhesive. I usually end up getting that everywhere though because you rely on the slipperyness (if that's a word) to get it back into place.
hope this helps,
Leigh