Your source unit/head unit is the heart of your car audio/video system. Your car will dictate what size radio fits into it. Radio come in two typical sizes, single and double din. This is the size of the opening on the vehicle. Some vehicles have had alternative sizes, but today the most common are the din and double din. After determining the size stereo that fits, you will need to determine if you want a audio only or audio/video combination unit. While most states don't allow the driver to view a video source they do allow the driver to view a data screen, like GPS. Once you decide the capabilities on your source unit, you now need to look at it's features. Do you want; IPod connectivity, GPS, Bluetooth, HD radio? Most aftermarket source units allow for external amplifiers, so that shouldn't be a concern unless you plan on running a multitude of external amplifiers. We install tried and true head units from Kenwood, Clarion and Sony. Stop by our stereo store in Pittsburgh to discuss the source unit right for your vehicle and lifestyle.