If you have had that leak for a while, you will probably find a rusted window channel under it. In that case a new seal and/or gobs of goop is a temporary fix at best. Repairing or replacing a window channel by a good body shop is less expensive than you may think.

I had the rear channel on my 300CD fixed and installed a new seal over that. Five years so far and fixed. I had a glass shop R&R the rear windshield since coupe glass is very hard to find. As I recall, the whole job came to about $450.