Since most people carry music on their smart phones why not just replace the car stereo with the Devium Dash Platform*. Then one can use their iPhone 3gs/4g/4gs or ipod touch as their stereo unit or navigation system and not need a pricey dealer installed option as well.
Msrp 349.00 to 389.00 depending on type of faceplate.

*Link deactivated due to malware warning.