Music via USB in car issue!

[kmcdwyer]

So I have a galaxy s2 with about 125 songs on a memory card. I also have downloaded an additional 25-30 songs using the Amazon MP3 app to my device as well. I want to play all the music through my radio in my car via the USB port available in the car. I used to be able to do this effortlessly with previous android devices, but with this phone I am hitting a snag. When I connect the phone to the car, the only music that plays is the 25-30 songs I downloaded from amazon. I cant figure out how to get the music on my memory card to play as well. The only way to get the larger list to play is to connect to the 3.5 mm jack my car also has. I don't want to use this method though as I cant control the music with my steering wheel controls like I can when I play through the USB port. Anyone have any ideas as to why this is happening and what I can do to access ALL of my music via the USB port? thanks in advance for your help!!