Just wanted to share an app I found that lets you control Pandora, Spotify, and other music apps with your Avic. It is called Music Controls Pro and is $6 in the Cydia app store (have to be jailbroken). I have used it in the past and it was alright, but the latest version now displays the track title, artist, and album art on the Avic screen. It is really cool.
Here is the official page, but the easiest way to get it is by searching for "music controls" in the Cydia store.