Basically, I'd wanna set it up to get power from the USB, plug into the AUX jack or use blue tooth, have it automatically power up with the ignition and have it pick up where it last left off. It needs to remember all the settings without needing it's own battery.
An equalizer and better bass would be nice. If it can stabilize the volume from one sound file to the next, that would be great. It would be permanently left hooked up in the car. Random mp3 selection is best. Is there a certain brand or model that works best?
And, does blue tooth make for better sound quality than the AUX input jack, or does that make no difference?