All switched ignition circuits are powered by the ignition relay. Not shown in Advance Autowire on-line version or Haynes, their 'late UK market' diagram is for 1978 and later. It's not known exactly when that version came out but Geoff's Oct/Nov 77 built 78 model has it so possibly part of the wiring changes for dual-circuit brakes on RHD cars in May 77 at chassis number 436465 (GT) and 437181 (roadster).

Note: This is one of three multi-way plugs behind the dash.

August 2026: Note that if the ignition relay fails no ignition-powered circuits will work. The engine will start when you turn the key to crank but cut out again as soon as you release it, this is because the coil has an alternative power supply during cranking from the starter solenoid.

• There are two separate green circuits - the original circuit fed from the 2nd fuse up in the 4-way block for all the original green circuits, and another fed by an in-line fuse from the white/brown circuit on the relay output for the cooling fan. This fuse is located below the fusebox, probably with the hazard flasher fuse which has brown wires both sides.
• There are three separate white circuits - one from the ignition switch feeding the ignition relay, another in the rear harness feeding the fuel pump, and the third in the dashboard sub-harness feeding the ignition warning light. These last two are taken off the white/brown circuit from the relay output.
• The schematics show the original rectangular Lucas relays with W1, W2, C1 and C2 terminal numbering, but in practice cylindrical relays with ISO numbering were fitted. The relationship is as follows:

Information on the ballast resistance can be found here.