This just occured to me, the answer is not personally important as I have an upgraded HU (alpine 7894) but the question has been posted before and I didn't see an answer. If you use two addapters to convert the Ai- buss to RCA and then get the adapter for the stock HU will the system play at all? There was a thread on this starting last spring but I didn't see an answer.
What just occured to me is that the Alpine MP3 CD changer may not have the electronics necessary to extract the audio from the tracks. As this model was ment to be used with specific after market HUs it might have been decided to just put the decodeing algorthurim in those HUs using the bus and not in the OEM HU.
There is no indication that the MP3 player was intended to convert to FM modulation or to be used with a casset player Alpine may have saved the cost of the converter for MP3 and also to convert from red book .wav files into normal audio.
Andy