1985 90 REar fog light help needed

I've done a search.
I cant seem to get my rear fog light working:

The light itself works, i attached it to another circuit and it lights up fine.

The switch works, electric tester shows a through put when switch turned on (buzz sounds).

I am pretty sure its connect right. Yellow and red wire from light goes to red and yellow wire (2 on the connector) from the loom.
Black from light to ground on loom.

I have triple checked everything, reconnected every twice and still no luck. I just dont know what to try next, any ideas?

MOT is comming up soon and i am wondering to just put new power line down directly to the light from the existing switch.

Chars.

You do know the fog light only works when the main lights are on?

Yer. I see its wired to the dip of the passanger headlight on the lr diagrams, i have changed the headlights earth to come from the engine block rather then the loom and the lights are so much better. I also tested this had an effect by putting the headlight earth back onto the loom, no change. cheers

might be a broken wire in the loom? Sometimes the loom that runs through the chassis can rub on the various bits and pieces that's inside the chassis.
Have ye tried flickin' the headlight master switch on furst and then the fog light switch (as said before)

Yer.. I know it doesnt work without the lights on. I dont know if it works with just sides on as never tried when it did work so have been trying with main lights onn dip. I have herd the loom could break, thinking about it again now you say it the last couple of k have just been doing offroading. Is that possible at 25Kmiles? Cheers