ok, I meant to get back to this earlier ...
You have two basic options, you can either get a headunit that has this type of input or splice into the speakers wires at the output of the radio and add it there. If you choose option 2, you WILL be driving the car's stock speakers with the output of the ipod. As stated above, I seriously doubt you will get any volume that way, those outputs are preamp/headphone outputs. The only other option is to get a preamp with a 2 inputs and a 4 channel amp - this is the expensive route. I don't think there are any other ways short of taking the headunit apart, finding the preamp inputs and making some kind of connection there.