Haha that actually gives me an idea. Replace the radio screen with some sort of puzzle/etc and tell them once they solve it then the radio will come on. Of course it will only lead to nothing :P
Haha! I'd love to see that happening ... wait I can enact it now:
zillacles: Hey, check out my car PC .. it's got everything: DVD, hard drive full of MP3s, GPS, Internet access, weather, Bluetooth, wifi, OBD ...
Passenger: Wow! That's awesome ... *fiddles with touchscreen* wait, how do I turn the radio on?
zillacles: err .. yeah, about that ..
Passenger: You DID say it has everything, right??
zillacles: .. well, you can't demand the radio be put on ..
*passenger jumps out of car whilst in motion*