Does your truck's stereo support Bluetooth audio? (A2DP) Audio is a separate feature of Bluetooth than using the phone aspects of your iPhone.
If you truck has supported Bluetooth audio in the past, but now doesn't, then that's a different matter. If that's the case, did this change for you with the update to iOS 6? You may need to remove your device from the Bluetooth "installed devices" and re-install it.
I was able to play music through it for a while. Once i update to ios6 the airplay button dissappeared, but still played music through the stereo however now it has quit. When I attempt to play music the truck will start the music playing through the phone speakers and I can change the songs from the truck but it will not play through the truck its self..
I see you're using ios6... I'm having issues with usb playing music through my truck... What I have to do to get music to play is actually shut the phone off by holding the powerkey on top and sliding the shut down button. once the phone is shut down, press the power button to turn it back on. Once my phone powers back up, it will start playing music again.
Let me know if this works for you.
It's so freaking annoying and ios 5 worked fine, but once I upgraded to ios 6, this problem began. Hope it helps.