A central locking system has now almost any vehicle. They can be installed easily into existing cars, if there is some experience in dealing with the on-board electronics.
When installing the existing rules are observed in order not to jeopardize insurance and registration / TUV.
A central locking system is usually supplied as a kit with schematics and wiring diagram.
Some models can also be operated by radio
Here links to Central Cars

Here you may find Circuit, Basics, Schematic Diagram and DIY Kit components for Car central locking