Engadget
A Bluetooth adapter may be all you need for wireless audio, while a CarPlay or Android Auto display can give an older dashboard a much bigger update.
An older car doesn't need a new sound system just because it was built before Bluetooth, CarPlay and Android Auto became common. If the radio and speakers still sound fine, you may only need a small adapter to enjoy music from your phone. Plus, a portable dashboard display can add navigation and other phone controls without replacing the factory stereo.
The right upgrade depends on what's missing. If you just want wireless music or podcasts, start with the connection between your phone and the stereo. If you want maps, calls and messages on a screen, a portable CarPlay or Android Auto display is a bigger step.
Replacing the head unit makes more sense when you want those features permanently installed.
Sound quality is a separate issue. If your phone already connects the way you want but the system sounds weak, distorted or short on bass, the better investment is likely speakers, an amplifier or a subwoofer instead.
If your car stereo has a 3.5mm AUX input, usually a small round socket labeled AUX or AUX IN, then a Bluetooth receiver is likely the simplest upgrade. Your phone connects to the receiver over Bluetooth, and the receiver sends the sound through the car's existing AUX input.
You tune the car radio to the same frequency to hear it.
A wireless Android Auto adapter only works with a car that already supports wired Android Auto.