Im not sure if you can but i was researching this until i realized that i wanted to buy a new in dash setup. But when i was reading up on this it seemed there was no in dash auxiliary, so i was reading up on those radio modulators for ipods, but not the ones that are notorious for being horrible and a waste of money. Some one was talking about one where you can hook up the modulator to the antenna of your car for a better signal or sound.
Sorry if this doesnt help just thought id post since no one has yet.