12V Automotive Changeover Relay 40A with Socket Holder

Exactly what was needed.

When it comes down to it, it's only a changeover relay like so many others that can be bought from a variety of car stores etc. However, it's a well enough made item that might be at least a little bit resistant to the lighter splashes and general messiness that can exist even in reasonably well protected places under a car's bonnet. It's also nice to have a plug that fits on to it (rather than using individual spade connectors) because this gives the job a more professional look once everything is wired up. I saw a comment saying that sometimes these don't have the wiring diagram printed on them and that someone wasn't sure what the middle terminal was for. Just like the picture, mine did have the diagram printed on the top and wiring is with standard relay terminals. Even if the circuit diagram is not printed on top, there should be little numbers embossed onto the plastic beside each terminal. Terminals are: 85: connects to the low-current switched source on the vehicle (e.g. the power feed to headlamps/horn/whatever) 86: To earth/car body (completes the circuit from 85 to switch on the relay) 30: High current source (e.g. direct from battery with in line fuse) 87: high current line to new accessory (e.g. spot lamps/air horn/whatever) 87a is on when there is no current coming from 85 and off when there is. If the relay is not being used as a changeover, this terminal does not need to be connected (although you can just get a switch relay with no terminal 87a)