I cut a new seal from black rubber sheet and finished the job with a carefully applied smear of sealant. Ultimately it's a bodge but it does the job and you'd have to be a bit keen to spot it even if I do say so myself! Might also be able to something similar with an o-ring or some other type of seal.S.M.
I doubt a water leak would allow the windows 2 steam up.... I would say your heater core is leaking