Just about every type of vehicle (cars, trucks, and boats) has relays that control amperage circuits. A great example of relays at work is the headlight system in a GMC S15. The headlight switch generates too much amperage for the headlight wiring. The relay acts as a go-between for low amperage circuits and high amperage circuits.
Relays are versatile and can handle several processes at a time, and this leads to smooth functioning throughout the vehicle. Unfortunately, it also means that when one relay fails, you may find that it affects multiple processes. When the headlights aren't working on your GMC S15 or the radio isn't operating right, it could be a worn relay.