The indicator relay is a small aluminium can which houses a switch whose contacts open and close at specified times which will cause the indicator lights to flash. One of the contacts is connected directly to the metal frame inside the aluminium can while the other is connected to a bi-metallic strip which is isolated from the frame and is in turn attached to the 12v supply. A small heater-coil is also attached to the bi-metallic strip and when the indicators are operated current is allowed to flow through and light the indicator bulbs. As the current flows the bi-metallic strip bends, this opens the switch contacts, causing the lights to go off. As the bi-metallic strip cools down it will unbend, causing the contacts to be made again. This series of flashes will repeat until the indicators are switched off.
The indicator relay switch is usually to be found close to the fuse boxes. A faulty relay switch can cause the indicator lights not to flash or stay in the on position. A faulty bulb will cause the rate that the indicators flash to change or fail to operate.