https://www.youtube.com/results?search_query=how+to+use+multimeter Just pay attention to when they talk about checking for continuity and voltage. Don't worry and amps or resistance for now. You'll need resistance later when you start testing sensors components and you will probably never need amps when working on your car. Your car is 12 volts DC!.
https://www.youtube.com/watch?v=RtNMhCzq72M
Put it in reverse, disconnect wires, test the switch to see if you have 'continuity', and go from there. That test will tell you if it is the switch or the wiring.