Older cars can gain modern connectivity without replacing the entire sound system. Bluetooth receivers are suitable for cars with AUX inputs, while FM transmitters work with radios lacking AUX; portable CarPlay or Android Auto displays offer navigation, calls and messages without altering the factory stereo. A replacement head unit is more appropriate when these features need to be permanently integrated.

The best upgrade depends on the missing capability and the car’s existing equipment. AUX Bluetooth receivers generally provide a simpler, less interference-prone connection than FM transmitters, while wireless CarPlay and Android Auto adapters only work with stereos that already support wired versions. If connectivity is adequate but sound quality is poor, upgrading speakers, adding an amplifier or installing a subwoofer is likely to be more effective; replacing the head unit may also require specialist wiring to retain steering-wheel controls, cameras and vehicle settings.