Brake Light Relay

Any automotive accessories relay should be suitable. You may be able to get one that includes the diode but this is not usual and you will have to add one. In both cases it is essential to get the diode connected the correct way round or you will blow the green circuit fuse, possible the diode, and almost certainly your new brake light switch. As shown it is correct for negative earth systems the older positive earth systems will need to have it wired the other way round. The diode must oppose normal current flow in the circuit as the only time it conducts is in the presence of back EMF from the relay just as the brake light switch contacts open, and that current is in the reverse direction from normal. The diode 'shorts out' the back emf and prevents the brake light switch contacts from sparking and welding just as they open.

If you mount the relay close to the switch you pick up the earth from its mounting, and if you use a connector to pick up the green circuit everything can easily be restored to normal very easily if required. But if you connect terminal 30 to the purple circuit at the fusebox instead of the green or via an in-line fuse to the brown, your brake lights will probably be slightly brighter and you won't suffer from the indicators slowing down when the brakes are applied.