Help Rear lamp issues 86 Atmo

Hi all
I have suddenly encountered a bug in my 86 Atmo.
the lights work fine, high and low beam, front and rear. With the lights off, the trafficiators work fine.
But if I have the lights on main beam, and I flick the left trafficator on, the right rear tail lamp and trafficator flash, and the left rear tail lamp flashes while the trafficator on that side flashes dimly.
If I flick the right trafficator on, the reverse happens.
And if I press the brake with the lights on, the both rear trafficators light up, and If I also have the trafficators on, and press the brake pedal, the trafficators stop working.
Which suggests to me there is a break in an earth circuit somewhere.
I have cleaned the two large plug/sockets under the cover beneath the cooling duct on the firewall forward of the engine, all of the plugs I can find at the back, the two large ones undereath the steering column, and the single large multipin just forward of the right door.
Sadly my workshop manual is in German - my English spare is up at my sons in Auckland at the moment where he is working on another atmo.
Any ideas?

I have seen this sort of outcome where a towing electrics has been wired in badly.

No such work on this car.
I should have my English manual later today, but meanwhile will keep trying to figure out what is going on.
Bound to be a simple bad connection somewhere, although checking all the lamps at the rear of the car, all of the earth connections appear to be intact.

Graeme

This type of thing happens when you loose the earth to the Stop/Tail lamp. This is because there are two voltages (stop & tail) which have a common earth ...... if that earth fails one or other of the voltages goes straight through to the other parts of the rear light cluster.
Suggest cleaning stop/tail bulb holder and confirm by meter the metal of the holder (the part that you plug the lamp twists onto ...... baynot fitting) is connected to earth ........... also check the two Stop/Tail lamps as its filament could have an internal break to the casing (earth).
I hope this makes sense.

I agree completely with John on this one, sounds like a broken earth...and on the atmo the rear light loom runs across the top of the engine and into a plastic wiring box in front of the engine on the bulkhead. I assume the earths go the same way. I think there is a connector on the rear bulkhead below the tailgate and parcel shelf too.

Good news guys, and thanks for your help.
I had cleaned all the connectors etc and earth checked continuity between the tail lamp assemblies and the block.
Following your advice, I got a bit more scientific. Fist of all I jumpered an earth from the block to the tail lamp wiring and problem solved.
I then worked my way back through the system to one of the two plugs on the bulkhead.
The earth was absent at that point, so I traced the earth wire back.
Then I spotted the problem.
There is an electrical block device that connects to the distributor and sits in a bracket bolted to the rear of the head adjacent to the cam tunnel blanking plate.
The bracket is held by a single bolt, which also serves as the earth point for two wires.
One of those wires goes back to the relevant multi-pin plug/socket in the little box on the firewall!
That wire had parted company with the earth lug.
Replaced the lug, bolted her up and whoopee - sane rear lamp behaviour once again!
Back on the road again!!! And loving it.

great news....!