It took a while of snooping around the car and finding where the water comes in, but both Jack Crosby and I (and a few others, I believe, not including IM owners) have managed to make our cars reasonably water-tight with the top up and side windows in. We both documented the fixes on here, too. These are usually VS or CMC cars, but it applies to JPS, too, which have their own set of water abatement issues. It involves weather strip in some places, expanding foam in others, sometimes a combination of both, but it can be done. I don't believe water intrusion has been an issue with IM or Beck cars (but since I drive a CMC I can't really speak to those).
I've driven in some heavy downpours over the years and now have it sealed up such that inside fogging began to be an issue but then I added a gas heater that flows the air through, rather than recirculate it and that cured the fogging issue, but that's an expensive fix, for sure. I don't have defroster vents, just the flow-through heater but it works.