Its a male RCA to male 1/8" / 3.5mm stereo mini jack. Many cars you can see the female RCA's in behind the radio. I would suggest its a DIY for most people.
Get a radio shop to connect a cord to the Mac that is compatible with your I phone plug, on accessories. That's the best low cost solution, sounds great.
I can just see you draggin' those down the road, blasting your devil music Don't roll the window all the way up or you might cut the wires and lose your speakers
Haha! Just a demo, but in person, they make those Klipsch sing! That just might be the most expensive McIntosh system, that is if I parked it in my living room! Just a fun experiment.