It really doesn't matter how well you ventilate - when the relative humidity is 100% - nothing is going to evaporate. The only things that have worked consistantly for me are:
(A) In warm weather accept that you're going to get wet and live with it.
(B) In cold weather slow down so you don't work up a sweat and have spare clothes available.
(C) Really heavy rain is much easier to deal with from inside a rest stop.