Strange electrical issue

1974 Series 3 petrol
Red driving lights at rear and number plate light sometimes don’t work. The Parking Lights work, brake lights and indicators work but as soon as I switch on the headlights the rear lights go out.
It seems to be an intermittent problem, twice this week it happened at night, the first time they came on after I had pulled over. Second time I pumped the brake pedal a few times to see if brake lights were working (they were) and hey presto on came the rear driving lights.
Last night I took off the inside rear corner covers to look at the wiring , fired her up an all lights were working, turned her off and on again = rear driving lights not working! The bulbs look fine
Is there a logical step by step process to troubleshoot this? Thanks.

Pop the steering column cover off and check the wiring around the fuse box - the contacts may be corroded

Do the front sidelights still work? if not check the wiring behind or replace the head/side switch. If the rear lights fail independently of the sides check the wiring to the rears - there are bullet connectors behind the panel in the rear right corner iirc.

First place to start is the earth connection for all the rear lights. That's one thing that didn't give me a problem on my SIII, so I don't know where it is.

The wiring goes from the front right door, back along to the right rear lamps, and then across to the left rear lamps.

All of the lights at the back have a coloured wire delivering power, and then a black one to connect to earth. These blacks join together on the left side, then the right side lamps black wires join that. I'm fairly sure that all happens inside the right-hand light cover inside the rear tub. They should all connect to a stud (of some sort) on the body behind that panel. Take the connection apart and wire brush everything to clean bright metal. Coat with Vaseline and put it back together.

Your lights are now earthed to the body. When the car left the factory that meant the whole body. You've now got years of crud and corrosion in all the connections between the body parts, so if your lights aren't fixed by the last paragraph, this is where your next job is. The simplest fix is to make a connection between the tub and the chassis with a decent bit of wire.

Popped steering column cover; all looked ok.
Side lights still work.
Yes there is a four earth stud behind the panel. I will clean it in the morning and let y'all know how we get on.
Thanks for the help.

Thanks Guys, that worked a treat. All the black wires (terminate in brass bullet connectors) go into a cross shaped terminal on the right rear side behind the panel.I took everything apart and back to bare metal.Everything is now working as it should. Thanks again for the pointers.