An article from Popular Mechanics:

Required Hardware: A smartphone and a stereo with an auxiliary jack

Cost: About $35, not including the smartphone

Setup: Smartphones, such as the iPhone or those running the Google Android OS, are like handheld infotainment systems: They play music, have built-in GPS nav systems and even understand voice commands. The trick is making them convenient to use in the car. For our iPhone, we used three parts: a $10 mount, a $20 remote mic/Home button and a $5 auxiliary cable to connect the phone to the car stereo. When prompted first with the Home button, the iPhone responds to voice commands to find music (“play Ted Nugent”) or retrieve numbers from your contact list (“call Ted Nugent”). Most Android devices have similar functionality. Don’t know your phone’s voice commands? There are smartphone apps that catalog the standard ones, and programs that let you customize (although you’ll have to jailbreak the iPhone). Smartphones can also stream online radio, such as Pandora-naturally, they mute music to allow for incoming calls.
The Bottom Line: Not the tidiest method, but capitalizes on the extremely powerful smartphone.

Read more: Build Your Own Car Infotainment System – Popular Mechanics

Easy Peasy,