Interesting electrical problems with lights and heated rear screen: Any ideas?

For a couple of weeks now, when I put the sidelights on, I get the heated screen warning lights too. No voltage at the rear window itself, just the warning lights. If I actually put the heated screen on, the sidelights come on, together with the little green 'lights on' warning light on the instrument cluster.

Now of course when this sort of thing happens, everybody says 'earth'.

But I'm not sure about that. If I put a temporary cable directly from the rear screen terminal in the back window to the battery negative terminal, the symptoms are the same. If I pull the heated rear screen fuse, the symptoms are the same. If I pull the rear screen relay, the symptoms are the same.

At the rear of the vehicle (a 2006 TD5 Defender) the black (earth) wires all disappear into the wiring loom and into the chassis. There's some earth wires come out of the chassis rail under the driver's seat and bolt to the side of the transfer box, which I understand are the earths from the rear. All cleaned up and put back together. My meter says zero resistance between the black wire terminals in the multiplugs and the battery negative terminal, and between this and the earth point on the bulkhead.

I've heard about a voltage sensitive switch (to prevent the heated screen draining the battery when the engine is off) going wrong and causing warning light issues, but this does not appear to be fitted to mine - it wasn't present on all models.

So I'm starting to think about some sort of a short between the heated screen supply ('upstream' of the relay and fuse) and the lighting circuit.

Any ideas folks?

Well, its sorted itself out spontaneously. The little warning lights have gone out and only return when I press the button. I can no longer use the heated rear window switch to turn my lights on either . I don't know what's different. I fitted a new switch on the transfer box yesterday, to activate the centre difflock warning light, but it's hard to imagine how that could have changed anything.

Don't you think it's more likely that you have disturbed something replacing the switch than you having a self healing "issue" within the loom ? - I don't know the TD5 loom - but, any relevant joints kicking about where you were working ?

I would'nt be happy to trust self healing on any landy, a battery cut off and extinguisher might be a good idea.

Well, I'm not suggesting it's 'mended' in any simple sense. Yes of course things might have moved while I was fiddling about over the last few days but the symptoms resolved themselves sometime after everything was back together. I'm still inclined to think it's an intermittent short between the warning light wire and one of the lighting wires. My external lights are LEDs so it only takes a tiny bit of power to illuminate them. The TD5 loom is big and bulgy and packed with millions of wires. The dash area and behind the centre fusebox is very congested so it would be easy for things to get pinched or chafed.
Yes, I have a fire extinguisher and it doesn't take too long to disconnect the battery.

for reference

Now that's handy - a diagram of just the earth wires.
Is there a corresponding one for the earlier TD5 model I wonder?