Having just installed a replacement Onstar module (VCIM) with Bluetooth, I needed to be able to use the A-pillar microphone while driving with the top down. With the standard setup, trying to make yourself be understood at anything over 30mph is an exercise in futility.
So while messing around I found that cupping a hand around the mic improved things and so I tried a number of different wind shields made from cardboard and such, but ended up finding that a piece of open-celled foam over the mic slots works as well as anything! With this, I can now use the mic at up to 55-60mph (top down, windows up) and only after that does the wind start to make your speech break up.
You can do this in any number of ways. I used a piece of 1/2" gray foam sheet that I kept from some equipment packing. I cut out an oval shape and rough-sculpted it with scissors, then smoothed it with a sanding block. Really didn't take long. Because my foam was gray, I also lightly sprayed it with flat black paint, but that's optional. Then, to install, I just stuck it over the mic area with some 'extra-sticky' double-sided sticky tape. Easy-peasy.
Alternatives to the packing foam might be to use the top cut off a foam musicians mic shield (I saw them in Best Buy for $4), or even the foam earpiece off an old pair of headphones. I doubt that the shape matters, just that it well-overlaps the mic area. Also, make sure your tape doesn't bridge and block any of the mic slots.
Here's a photo of mine in place (driver-side A-pillar):
[attachment=1]
