If your car has cd-changer controls, you can look for something like "USA Spec iPod® Interface". for around $130. It's the old connector but also works with Apple's adapter for the newer lighting connector. It also charges your iPhone and you can use the steering wheel controls to change tracks. There's also an analog RCA input, if you have other devices you wish to use. This does sound much better than the transmitters for very little more.
If your car is new enough, you might check out the dealers to see if they offer anything, it will probably cost closer to $300-500 but would be better integrated.
Jim