Mercedes Multifunctional Relay Available Here. You car requires a lot of electricity to operate. In order to handle all that power without taking up a lot of space, car builders use a multifunctional relay to help control the flow of current between the battery and the equipment. This relay allows a small amount of current to switch on and off a device that normally requires a much larger amount of current to operate. Without this multifunctional relay, your car’s engine compartment would be chock-full of huge switches to turn on things like your headlights and wipers, which would leave no room for the important things like your air conditioning unit. With this relay, those switches are much smaller, and there is plenty of room for your air conditioning unit.