Use new, warm rubber! Cut old rubber all around with razor knife & pop glass out. Dish soap is good installation lube. Put new rubber on glass, & small cord in outer groove with cord ends hanging out together at bottom. Set in place on lower edge of cab opening, with as much of the "flange" in the groove as possible, and ends of string inside cab. Working slowly, pull cord out, which will spread open the rubber groove, and push glass/rubber onto flange as you go. Obviously, this is much easier with an extra pair of hands. Hopefully, it's all in place when you get around to the other end of the string!
cm
If you can't fix it with a hammer and screwdriver, you need a bigger hammer. 1965 Chevy C10
I'm getting ready to do the front glass, and am a bit confused by "small cord in outer groove with cord ends hanging out together at bottom" can yu explain a bit more?
I bought a 50' spool of 3/16" smooth nylon cord. It's just handy to have, and works well for this kind of thing. The rubber gasket has two grooves: one each on the inside and outside. The inside groove fits on the outside edge of the glass. The outside groove fits the "pinchweld" flange around the window opening. Install the rubber on the glass. Starting in the middle of the bottom, push the cord into the outside groove all around, leaving about 6" "tag" ends of the cord hanging out together...these are to get ahold of. Set window in place with cord ends inside, and engage flange in groove as much as possible. Pulling the cord will spread the rubber groove over the flange. Work both ways on big windows, pushing and nudging as you go, until the last bit of cord pops out and the window is in. For the guys at the glass shop, it takes about 3 minutes. It took me and a helper about 15 minutes, and we had to start over once. Good luck.
cm
If you can't fix it with a hammer and screwdriver, you need a bigger hammer. 1965 Chevy C10