Just use an 1/8 to 1/8 auxiliary cable (available at RadioShack or any electronics store) and connect it from your phones headphone jack to the car stereos aux input. Some cars come with bluetooth built into the stereo, and if your car has it, check the owners manual to find out how to pair it with your phone.

How do I connect my phone to a speaker?

The simplest way to connect your Android to a stereo involves a 3.5 mm headphone male to two RCA male Y-cable. This cable will split the outgoing audio signal from the Android into right and left channels with RCA plugs. Plug the 3.5 mm end into the headphone jack on the Android.

What can I use if my car doesnt have an aux cord?

If your car or radio doesnt have an auxiliary input, youll be better off with an FM transmitter. Effectively, the FM transmitter of today is a Bluetooth receiver, but instead of sending the audio to the stereo via an auxiliary cable, it broadcasts it over an open FM radio frequency.