LOL! 99% of cars don't have sound systems that would let you hear the difference between a CD and an MP3 so your best bet is get a flash drive or an ipod and hook it up digitally and letting the deck do the A/D conversion and call it a day.
To the OP: You can't get that without spending a ridiculous amount of money. Namely because you'd have to buy a CD player out of a car that has a built in ipod interface and put that into your car. Most people just buy aftermarket decks and live with that.