Rear Fog Lights, Factory-fitted, 1980 on
Hover over a wire to confirm the colour

Note that the rear fog-guard switch needs a tell-tale warning of some kind to pass the UK MOT. This can be as simple as a coloured tag only visible when the switch is in the operated position, or a tell-tale light. As there appears to be no external tell-tale (Clausager doesn't mention one and I have not seen a diagram for this year) on the face of it that would mean two bulbs in the switch - one for the night-time illumination that all the 77 and on rocker switches had, and another for the tell-tale. And from Stephen Aitken on the MGOC Forum this is indeed the case - (a) above being the night-time illumination and (b) the tell-tale.