My phone.
The cable from the back of the radio comes out underneath into where the 12v power socket is.
I then plugged my phone into it with an adaptor ( because I have iphone5 ) that takes it from the iPod connection to the iPhone 5 size lead.
There is power to it as when I plugged it into my phone, the phone came up with that it was charging.
Confused!!!!
There is no mp3 button either.
Even though it says mp3 on top of the head unit.
Or am I being thick