There should a little square box next to the play button on the iPhone that lets you witch back. it switches to the newest connection.
Hooked up iPhone to MEDIA connector, worked OK for about 20 sec. Then quit. Finally figured when starting car, Bluetooth hooks up to phone & kills MEDIA connection. Only way to re-initialize MEDIA conn is unhook, rehook. Can't find any to re-initialize from RCD-510 touchscreen. Only option looks to be wait for Bluetooth, THEN hookup to MEDIA.
Could just use Bluetooth for everything, but phone gets kinda hot after transmitting a while. Also, MEDIA connector keeps charging phone with ignition off.