Relays are nothing more than an electromagnetic switch. There are two sides to a relay 1) Control side 2) Power side. The Control side has an electromagnetic coil in it that gets hooked up to controlling power source(in your case car alarm) and the other side gets a wire to ground. The Power side has a switch in it and gets a power wire, either constant or switch battery voltage and the other side gets a wire going to the component you wish to power(in this case the door lock). When your car alarm sends voltage to the electromagnetic coil on the Control side and energizes it, turning it into a magnet, which pulls the switch on the Power side closed, allowing the electricity from the battery voltage wire to flow to the component that is to be powered (door lock).
I hope this helps, and if you have any other questions, please feel free to ask. It's what we are here for.
Friday, April 26th, 2019 AT 11:11 AM