Two things spring to mind here.

(1) Have you got the air set on recirculate. This is the button just above the air con button which glows amber when set. This will mist up the inside of the car to dangerous levels very quickly.

(2) No matter how bad the interior misting is in my car, I find that if I switch on the air con and turn up the blower fan, it clears the misting in seconds. It's really that quick. For my money the interior demisting facility is the best reason for having air con, and in the S2000, I find it particularly good.

I don't think you should be having this problem at all!

 

Get a hardtop, that will solve your leaking problems ...

 

fixed the problem in my car!!! I push the switch all the way to close the roof. latch the roof, then pull the switch (like i was going to open it again) and that snugs the roof down onto the seal. took me a year to figure that one out, duh!