You can use speaker level to rca adapters to convert to rca, but it's less than ideal. If you are dead set on using the stock head unit, I'd recommend getting an amp that has speaker level inputs.
I didn't want to ditch my stock head unit, because I liked the look, but eventually I swapped in a nice looking Pioneer head unit that has Ipod controls, subwoofer outputs, aux input, and a ton of other features for about $190 (Pioneer DEH-P5000UB CD Receiver). I'm VERY happy with it - a great upgrade over the stock unit, and it looks really nice.