window channel rubber

[rddaves54]

I have 72 Chevy long bed, I am replacing the rubber in the window channels and was wondering how to do so without breaking the glass. The glass seems pretty strong but the new rubber in the channel does not want to slide on and when using a rubber hammer to drive it on it does not go on all the way down and I do not know how hard I can hit it.

Just got the truck back from pint and I am putting it all back together. Also putting on the side mirrors, does it matter where they are on the door? I was thinking just behind the side post to the vent window. I am putting the small square ones on, I like the way they look.
Thanks for the help!