Dim-dip relay
A few years ago, before the installation of day lamps, many vehicles were fitted with dim-dip lights. This prevented drivers from driving around on dark, murky nights on only side-lights, by turning on the headlights in a reduced voltage state.
When a former London taxi was bought in for its MOT, one of the failure points was that both front dipped beams were inoperative. Checking the bulbs, it appeared that both were in good order, but the condition of the dim-dip relay in the fusebox revealed the problem. The unit was red with rust and had clearly not been operating correctly for some time. A new relay was readily available from our local factors and, once popped into place, the dipped headlights were illuminated.