Update:

Fog lamp working normally again and not on by itself.

After removing the fuse box, the binnacle and other bits of dash to get at wiring I couldn't find anything amiss. But did notice that he fuse for the rear fog lamp was no6, not no1 - which when removed caused the fog lamp to work correctly?! - and was the fuse for the front fog light, horn, clock and interior light. But fuse no1 also had a feed for the trailer socket. So the fog light was getting power somewhere from one of these two feeds as it went out when fuse removed.

I gambled that the thicker of the two purple wires would be the feed for the trailer and cut it. Problem solved, fog lamp only on with dipped beam when fog lamp switch is on.

I also unwrapped some of the rear loom to the socket and the fog lamp behind the rear crossmember but couldn't find anything wrong. Rewrapped the loom with self amalgamating tape. Perhaps there was a short inside the socket? I don't use a trailer and have never needed the tow bar socket so happy with this 'fix'. But I'd like to know what actually went wrong so suddenly?

I'm still trying to get my head around this!
This is the fuse box layout I have:


Left bank top (no1) is front fog lamp/clock/horn/courtesy light; left bank bottom (no6) is rear fog lamp.
Removing fuse no1 cured the permanently-on rear fog lamp but lost horn, courtesy light etc. Fog lamp worked correctly in conjunction with the dipped headlights and switches.
With fuse no1 in and no6 out the rear fog lamp stayed on.
So fog lamp feed getting power from a permanent live related to fuse no1 but after fuse no6.

The wing diagram for the model year/type suggests that there is a feed via a purple wire for pick up point for auxiliary trailer (67) although not fuse 1?! Cant see how it relates to the horn, clock etc but it was the only fuse with two purple wires.

Cutting the feed to 67 cured it and all working AOK.

So, got to be a fault in the rear loom/trailer socket allowing perm power from 67 to power the fog lamp feed after the fuse.
What you reckon?

Check the towing socket - I had a weird light issue with a tow socket that had all corroded inside and in effective ‘completed’ the circuit - mine was indicators coming on with the brakes but same sort of scenario (one circuit back fed by another)

flat said:

Check the towing socket - I had a weird light issue with a tow socket that had all corroded inside and in effective ‘completed’ the circuit - mine was indicators coming on with the brakes but same sort of scenario (one circuit back fed by another)

Click to expand...

The trailer socket is covered with a tight rubber boot and under the socket flap the terminals look clean but I didn't take it off to inspect the wiring side.

A job for another day; as said I don't use a trailer so can do without it. If inclined I might investigate further and if find a fault in the wiring or socket, remedy it and rejoin the fuse box feed to the trailer socket.

cappers said:

The trailer socket is covered with a tight rubber boot and under the socket flap the terminals look clean but I didn't take it off to inspect the wiring side.

A job for another day; as said I don't use a trailer so can do without it. If inclined I might investigate further and if find a fault in the wiring or socket, remedy it and rejoin the fuse box feed to the trailer socket.

Click to expand...

Go to the cover on the rear of the tub & just disconnect the purple to the trailer socket.

as for trailer wiring I had a link wire in as most horse trailers only lit up 1 side of the lights.
Also on my own trailer I had a the purple live for the internal camera(but could turn this off when towing other trailers)