Not long after I bought my '08 Speed 3, I bought a new head unit from EONON that is made specifically for the Mazda 3. It is so much better than the stock radio and is fairly inexpensive. With the head unit from EONON, I have navigation, backup camera, MP3/WMV player, DVD and video file format player, bluetooth hands free phone and bluetooth audio and so much more. It even has an additional video output so when we go on road trips I can hook up an additional monitor so the kids in the back seat can watch the movie playing through the head unit.
Here's a link to one of the current models for a Gen 1 Mazda 3: