When a coil is switched off, a large BACK EMF appears across the coil. This back EMF may be several thousand volts in value, enough to destroy the SJB over time.

To deal with this we fit a diode at the relay, its low resistance short circuits the back EMF voltage and protects the SJB. You can get the diodes from any diode retailer, and they look like this: 

They are referred to as 4001 diode. You may find them listed as 628-9546 or 1N4007-E3 depending where you are looking. 

Observe that one end has a silver stripe on it, that is to ensure you fit it the correct way around. It is imperative they are fitted the correct way round. 

The diode goes between 1 and 5 on your Omron relay bases, bridging between the only feed from the SJB (being the light you have tapped from) and ground – with the black end of the diode going to the Nr 1 ground terminal.  Put another way, black end to earth, silver striped end to switching live from the light you tapped from.

Cover the exposed parts with some sheathing of some kind. I use household earth sheathing (like you might find in a plug socket) to cover the whole exposed diode. I am sure there is a politically correct colour that those who specialise in EU babble will know about – point being, just cover it up somewhow so nothing shorts out on it later on.

Advice from the Mustang forums recommended one type of relay made by Omron. I have tried others but the Omron ones are as cheap as any other and ideal for the purpose, so lets go with those. 

You can get them from anywwhere using the Omron numbers below. I get mine from here. 

The relays themselves are Omron G2R1SN24DC(S). The RS Online number is 353-821. They look like this: 

They plug into some special bases, which are also available from RS Online – they look like this: 

The Omron number is P2RF05E, the RS Online number is 353-944. Having plugged the relays into the bases, they slide on a metal rail in order that they can be fixed in the boot where you will site them. RS Online also stocks this, it is called DIN rail, and the RS Online number is: 424-131. That stuff looks like this:

When you come to wire up the relays, remember to fit your EMF protection diodes – this is critical. The relays have five terminals on the bases, this is how they wire up.

The relays are labelled thus: 

1 (-) 
5 (+)
4 (11)
2 (12)
3 (14) 

And they wire up like this: 

1 – to earth
3 – the live from the battery. 
4 – the new feed to the particular new light in question.
5 – the input from the source light you are tapping from; the switching wire if you like.
2 – redundant in this application. 

Therefore, the EMF protection diode goes between 1 and 5, bridging between the only feed from the SJB and ground – with the black end of the diode going to the Nr 1 ground terminal.