I have a Galaxy Nexus and I successfully connected it to the RCA Video Input on a AVH-P4300DVD. I just ordered an AVH-P8400BH. With double the resolution, it'll look insane.
You need:
MHL Adapter
HDMI to RCA powered converter box
5V DC USB Barrel connector for the converter box
Micro USB cable to power the MHL Adapter
RCA Component cable (Red White Yellow)
HDMI Cable
Get a Micro USB extension too because the MHL plugs right into the phone and because of the thick HDMI cable and microUSB connected to it, it's bulky and annoying. It's best to wire the whole thing cleanly behind the glove box and run the USB extension to your phone. That way it's clean, non obtrusive, and awesome.
Connect the MHL to the phone, the Micro USB (for power) to the MHL, and the HDMI to the MHL on one end and to the converted box on the other. Connect the 5V Barrel DC to your box to power it on and connect the RCAs to the box and to the back of your HU. The phone will automatically switch to landscape and mirror to your screen. You can play music while you display whatever you want on the screen.
TIP: If you wanna use your Nexus as a GPS unit, you can now download Google Maps to your device so if you're roaming or have limited data, your phone won't need to download the maps on the fly. You can download whichever territory you want and it even works in NAV mode. It's great!
Next step: figure out how to output the RCA to RGB and connect it to the P8400 RGB port (normally used for the Avic-U220 which I'm selling because it's garbage). They, everything will be perfect.