It requires a 2 amp USB power output. My guess is that your USB connection in your stereo for an MP3 player is 1-1.5. If you check the V4L site under "Accessories" they sells a 12v USB connector for your car so you may want to check that out. I'm not 100% sure but I think Radio Shack also sells them, because I've heard of 12v USB adapters with 2 ports.
Hope that helps.....