The back window glass kit is the seals between the panes of glass. It won't fix the leak you're talking about here. Ford used to make a rubber seal to fix the leaks caused by cracked/broken window frames, but they were crap. I'm not sure if you can even buy it any more. Ford also sells a "Foam seal" for the back glass but all it is is a butyl seal with a foam core. Go to the local parts store and get a roll of butyl tape and a tube of black RTV. Seal the frame to the cab with the butyl and seal the cracks with the silicon.