OK so this is what it's going to look like. The iPhone uses Airplay to transmit the signal to the Airport Express in the trunk. Then an optical cable connects the AE and an external DAC (digital to analog converter). Then the signal passes through RCAs to enter the MS-8 through the Aux-in. The main advantages of this setup are a) no wires between iPhone and trunk and b) virtually no degradation of the signal. Works great in the living room, now on to the install...(coming soon!)
P.S. I was originally not going to use an external DAC, and just use the headphone out on the Airport Express, but at only $30 I could not resist!