Transmitter, or go oldschool and use CDs. You'll probably replace your car after a couple of years if not earlier, whether it's to move up a pay grade or because you'll have some kind of repair that isn't worth doing, when you replace the car, the likelihood is your new one will be "newer" and probably already have some kind of aux input, so don't waste the cash on a new radio.
If you wanna go all in retro, get a speaker in the boot, run a cable round the interior lining of the car and voila, you have a DIY cowboy aux.