car speakers are designed to handle some moisture, and if your moisture barrier isn't intact it can cause a waterleak into the car, but you can cut a butter tub in half to make an umbrella over the speaker if you are concerned about it.
The majority of speakers mounted in doors will get moisture/water/dust on them. That kind of crap always gets in your doors.
My RE mids are in my doors and on the back side of the cone I can see spots where water has ran down them, a layer of dust on them ( gravel roads are a biach) etc. They still work fine.
If you are really worried about it the buttertub idea is a good one. Or maybe some of those speaker baffle things. They are like a little enclosure for your mids, they would keep dirt/moisture out, plus you would have to worry about sealin the door all up.
EDIT. Didnt see someone linked to those baffles already. Oops //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif