Which butyl sealer for back glass?

Interesting suggestion... how thick was the weatherstrip tape that you used?
I have tried a few, but
the best I have used is a "P" profile weatherstrip from Home Depot. It is white, soft foam. Don't remember the brand name. Comes as double roll in a cardboard package. Sticks well.
You can rope in the window without tearing it, and it seals perfect.
I have done many this way, and no leaks.