Looking for cheap way to hook up ipod to my car?

The easiest way to use your ipod is via auxillory port. if you don't have an auxillory port in your headunit tho, and all the speakers in your car get their music signal from a seperate amp then you can just get an RCA to 3.5 input like you said. That will let you send your music signal from the ipod to the amp. I suggest splitting the RCA's that you have running from the headunit to the amp, so that you can have both your ipod and stereo hooked up at the same time (thats how I did it).