My headunit, a Sony CDX-GT630UI, has a USB port on the detachable face, but not on the rear.
I got tired of being forced to keep the radio door open all the time while driving as I keep my iPhone plugged in & use it for music 99% of the time.
So I have it all torn apart, and am going to be soldering a female usb cable directly to the pcb and have it sticking out the back!
Now, if I can find my *$#%ing dmm I can start soldering =)
I'll post up pics when it's done!

 

like where this is going!

 

Could you have used a bent neck usb instead? You can then run the wire under the panel and have it come out under the radio door.

 

That is the route I was forced to take..
The Data +/- for the usb port did not trace back from the front flip-down face plate to the pcb inside the headunit. The usb connection on the faceplate is decoded into a chip that's on the faceplate's pcb. So no access to the usb data pins from inside the headunit, only the faceplate.
So I took apart an old usb cable & used just the metal end male plug piece & soldered a female extension onto it, wrapped it in electrical tape & called it good.
I'll get some pictures up when I get a chance..

Now I'm working on getting 6x8's in the doors. Although they fit most 5x7 locations so technically it's a 5x7 I suppose?
These are what are going in there.