How do I hook up my iPod to our 2003 Chevy Suburban?

We’re on a roadtrip and I’m trying to connect my iPod Touch to the onboard speaker system in our 2003 Chevy Suburban. I’ve got an iPod to RCA cable and the car has several sets of RCA ports, one inside the glove compartment and one at the base of the center console facing the back seats. We get sound when playing DVDs and through the radio, and we’ve been able to play sound through both these sets of ports previously. I think I’ve pushed all the right buttons and twiddled all the right dials. I’ve tuned the radio to 88.1 and turned on the sound system via the switch inside the center console. What am I doing wrong? Do these RCA ports not work for audio input?