A couple of guesses... I think with the '92 model year there was a problem with proper drainage of the plenum, beneath the windshield. Maybe you were getting water backing up there, and it worked its way behind the firewall, on the passenger side, to what you see now.
Second guess... Had the car been hit, had any bodywork done on it? Could be a seam opened up under the car, which could explain the wetness, and also why leaks are otherwise not visible.