RCA work well. I have the iPod integration kit in installed (kit described in this thread). Before that I had many RCA cables including Monster brand and other "low" priced cables that connected to the iPod headphone (1/8 plug) jack...I could not hear the difference.

Go for a cable that connects to the iPod via the connector at the bottom of the iPod instead of the iPod Headphone plug...the sound is so much better.. Check this link out http://www.provantage.com/cables-go-35509~7CBTY00J.htm for an example. I can't vouch for this one but since it uses the iPod docker connections at the bottom of the iPod the sound quality is very good .. of course charging the iPod is just as valuable IMHO and this one requires more of an install, maybe too much work http://www.discountcarstereo.com/detail.aspx?ID=896..