That's correct, an FM transmitter is probably the way to go, even though I have had bad experiences with them in the past, like bad frequencies, low sound and such. The only other way I can think of is to see if you can retrofit an AUX port in the back of your head unit, surprisingly A LOT of head units are actually prepared for AUX, and that would be a much better option in terms of quality, and at least you won't have to use the car's cigarette lighter.