There are 2 versions of this project, a 3.5mm input version, and a bluetooth version. I have pretty much stopped working on the 3.5mm version because bluetooth is better :)
My software skills aren't nearly as good as my hardware skills, so because of that, the module has issues. I think they're all timing-related. sometimes the module will "drop out" and the radio shifts back to the radio. I also don't have a good way to change the SID (Saab Information Display) to display custom text without the module dropping out even more than normal.
Some other small problems that need addressed are controlling the bluetooth module better; it has SPI built in, but the documentation for the chip doesn't describe any of the functions/commands available, and I don't know how to talk SPI, anyway.
Currently, the module is able to read "Next" and "Previous" steering wheel button commands, which in turn "hits" a button on the bluetooth module, telling the iPhone (and presumably Android) to change tracks. I also have a "play/pause" button available on the module. These features have worked in the past, however, currently the software doesn't work right, so it's very much a work in progress and those extra commands don't work.