Listening to music in the car through iPhone 4?

Basically, I have an iPhone 4, and Spotify subscription. I have a CD player in my car with no AUX. What's the best way for me to listen to my music through my cars speakers?
I'd prefer links please.

There's a thing at best buy that puts out a radio signal and you tune your car radio to that station. Its awesome. Like 50 bucks I think.

I have one of these

http://www.google.co.uk/...b&bav=on.2, or.r_gc.r_pw.r_qf.&biw=1280&bih=861&um=1&ie=UTF-8&tbm=shop&cid=16732497342085507647&sa=X&ei=vRNbUOaMMsPT0QXk6IGICw&sqi=2&ved=0CEgQ8wIwAg

Depending on the car you have, you can get an ipod car integration kit provided by gtacarkits.com
You can use the pick your car tool on the left side of the main page to see if your car is compatible. If it is, you can connect your iphone to the kit directly from the charge port. You will get cd quality audio coming out of your device and onto your car speakers, you will be able to skip songs using your car radio controls and it will also charge your device while it's plugged in. The kits are very affordable, ranging between $100-! 20, and are very easy to install. Most people have it in their car and working in less than an hour. The site again is gtacarkits.com

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