I've taken apart the stock mic housing and going to use it as my mounting point. Any suggestions on what material I should use to fill the housing around the new mic to seat it securely? I was thinking of using some foam tape....

Also, would blocking the back sound vents on the pioneer mic cause it to be too muffled?

So taking the stock mic out of the mount is really easy. There are two small phillips screws holding it in. You need a special screwdriver to take them out. They usually come in a set with three different phillips heads, plus some slotted heads. They are great for repairing glasses as well :)
http://www.the370z.com/members/plasm...502-195409.jpg
http://www.the370z.com/members/plasm...502-195440.jpg

Next, for my type of mic, the audio hole wasn't big enough, so I drilled it out using a 1/4 bit and rotating the bit around while I drilled. Ended up fitting like a glove
http://www.the370z.com/members/plasm...502-195448.jpg
http://www.the370z.com/members/plasm...502-201343.jpg

Finally some double sided tape to keep it in place and there you have it. The housing clips into the light assembly and your mic is nice and secure and functional. Will post reviews on how well it works :)

http://www.the370z.com/members/plasm...502-201328.jpg

Ok, so I ran into a problem with the mic I have. It was too long to sit properly in the stock housing, so modifications were needed...

http://www.the370z.com/members/plasm...426-202541.jpg
By shaving away the corner, but keeping the attachment point of where the light harness clips to, I was able to make it fit just right :)
http://www.the370z.com/members/plasm...503-071735.jpg
http://www.the370z.com/members/plasm...503-071745.jpg
http://www.the370z.com/members/plasm...503-071750.jpg

I drilled a pilot hole very carefully with a 1/4 inch bit, then used a very sharp exacto knife to shave the corner out.

And it's installed and working just as good as when I had it installed in the top right of my drivers side A pillar.