I got tired of dust/bugs blowing out of my heater vents when I turned the heat on in my 308 GT4, so I decided to check/replace the filter. Here is a photo of the heater core. Image Unavailable, Please Login
I replaced the filter with a foam filter from home depot that I cut to size ($0.99). Put everything together again, and tada. Much improved. Image Unavailable, Please Login
I'll be damned! I didn't know that there was a filter in there. What I did is use some standard furnace filter material and cover the inlet - right behind the grille. The grille holds it in place.