I know someone has probably done a write up by now but I thought it couldn’t hurt to do another with some photos for others to see what it looks like.
Buy a new cabin air filter. The cheapest place I found the replacement charcoal filters was http://www.Rockauto.com
for $18.86 (price is now $20.69). I ordered two so the shipping was $7.51 to CA. (Bosch "Cabin Filtration Product" part # c3861)
Use a flat head screwdriver to pry off the black plastic panel. It should come off pretty easily. There are four plastic snaps that hold the panel in place. Unsnap the power cord to the small floor light so you can remove the bottom panel completely. (You don’t have to do this but I found it was easier to get the old out and new filter in)
Unscrew two 5.5 mm machine head screws. Small plastic panel should fall off easily at this point. Remove filter, replace with new one and you just saved yourself at least $20.
I couldn’t believe how dirty mine was after 20k miles. All sorts of stuff fell out of it when I was replacing it. It was full of dog hair of course. I ran the air and it seems to blow harder now that the filter has been replaced. I think I am going to replace it more often from now on.