How to connect iphone to subwoofers?

So i have a 2006 saturn ion. The original headunit is apparently connected to the computer somehow and can't be removed. I talked to a friend today and he said he has the same problem and bought an aux splitter and an RCA to aux cable. He then connected the aux to both the subs and the original head unit and it works great. Can anyone explain exactly how to do it and exactly what i need for it.

You connect your iPhone to the headunit nothing else, If that doesn't work than your headunit is not compatible with an iPhone. The only thing your subs should be hooked to is an amplifier!