A relay is basically a remotely operated switch. A relay consists of make and / or break contacts operated by an electromagnet. An electromagnet is a coil of copper wire wound around a week of iron core. Usually one end of the coil is wound to ground, and a voltage of 12 volts is applied to the other side. Then a current flows through that coil, so that the iron core becomes magnetic and attracts a metal arm. There is a contact on that arm, which is resting against another contact in the rest position. As soon as voltage is applied to the coil, the changeover contact starts to move and then hits another contact point. The photo below shows an industry relay, with the coil wound from very thin copper wire on the left, and the contacts on the right.

The current required to close a relay is quite small, on the order of 300-400 mA (= 0.3 to 0.4 A) for a car relay. The contacts can usually switch about 20-30 A. With most relays this is on the plastic or, with MB, the aluminum house. Because the coil requires so little power, you only need a simple switch, and the length of the wiring from the control switch to the relay coil is no longer important. Another advantage is that the wire that carries the high current from the consumer can be kept as short as possible, because the relay can be placed anywhere. The manual switch, must necessarily be placed further away, in order to be conveniently operated .

The reason the headlights don't work through a relay is because it's a pretty hefty switch that can handle the current. The disadvantage is the extra length of wire to get from the front of the car to the dashboard. Loss occurs in this. A common modification is to give the headlights a relay.

The interior of a standard Relay looks like this (the contacts are not so clear here, but the switching current is still 20A.) The relay in this case is one with only a make contact, that is, that a connection has been made if there is voltage on the coil contacts. Fortunately, for the contacts there is a standard numbering (the DIN standard), which is applied to all German cars, and also to many other, but not to American cars / electronics parts. The coil contacts are always 85 and 86, while the coil is indicated in a diagram by a rectangle with a slash through it; this is also clearly visible on the house. The current should (usually) run from 85 (+) to 86 (-) Why this is the case will be discussed later. The contacts in a schematic are always shown in the idle state, so when there is no voltage on the coil. The contacts at a make switch are always 30 and 87. The supply voltage is always at contact 30. This can be directly from the battery, or via a fuse, or also from a voltage-carrying point via the ignition lock. If there is also a rest contact, it is numbered 87a.