Think simpler with the screen. Wire the screen to your car accessory switch and have it turn on and off with the key. Why waste power and attract onlookers with the screen on in the car while you are away from it?
Which HD radio interface are you using? The only one I know that works for the Mac is the RadioShark, and that's not HD.
Also, I think you can use your iPhone as a remote for the PB using RemoteBuddy.