While the Flex-Seal comment was a joke, I actually did fix leaky window seals back in the day by running a clean bead of black silicone around the outside and along the inside edge against the glass. I just masked it all off and used a wet finger to smooth it out before removing the masking tape and the finish again with a wet finger. No one could ever tell that I had done it and it stopped the leaks every time...even on cars with rotted window channels.