I would start by buying a new gasket for the rear window. LMC Truck sells them. To remove the rear glass, peel back the edges of the rear window gasket with your fingers while pushing on the glass. The rubber will start to fold over and go out the window opening. Then just keep working your way around the the glass and it will just pop up. To install the new weather strip slip it on to the glass first, then put a piece of electrical wire that's longer than the outside diameter of the window into the inner channel of the weatherstrip ( spraying WD40 into the channel the wire goes into is also helpful ). Set the glass in place from outside the truck with the bottom edge of the weatherstrip over the lower sheet metal lip. Have the two ends of the wire hanging in the inside of the truck. Have someone hold the glass in place from the outside by pushing on the glass. From inside the truck hold one end of the wire while pulling the other end of the wire towards the front of the truck. Have the person that's pushing on the glass stay in the same area you are pulling the wire out of. Pulling out the wire pulls the inner rubber lip of the seal over the metal lip the seal mounts too. When you have the glass out inspect the metal for rust and repair as needed. Removing and installing the glass is a easy job, it's just hard to write clear instructions about. If my instructions aren't clear enough you can probably find a you tube video that shows how to do the job.
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