Depending on your mag, there are either one or two gaskets on the cap. Have you replaced them? Does your cap have a crack in it? The coil may be weak and the moisture makes it worse. If you've tried all the above, get the mag rebuilt and your troubles will be solved.
Take an anti freeze jug and make a cover.
if it's a fairbanks mag theres a vent on the side it has a notch cut out in the top of it loosen the screw and turn the notch down and thentighten the screw the vent plate is about the size of a nickel with a screw thru the middle of it it's on the side almost right behind the big cap it worked for me on my 68 now i do it to all my machines
I was told to use some silicone spray and they said that will seal up the mag. just spray every now and then.
i've never heard that it's always a good idea to check the screws to make sure they are tight but dont over tighten you will crack the caps
It might help to wash the cap and plug wires with detergent. Antifreze residue and other grunge can make it easier to short out when damp.