Using Aux Input in a Pioneer AVH-5700DVD

I'm not sure what aux input you're using - but be advised that if you've plugged your mp4 player directly into the RCAs in the back that won't work - those are outputs, not inputs.

You need to use a Pioneer CD-RB10 (or CD-RB20 if you're running a cd changer, XM tuner, or any other IP-bus component) in order to get an auxiliary input.

If you've done that - you need to go to the initial settings menu and set aux input to 'on'. Instructions are in this link, page 89.