Today (2 months later) after the van having sat under torrential rains for a couple of days, it refused to start and displayed the same symptoms: glowplugs get on but a clicliclic noise and no start + funny warning lights going on by itself for a while.
I was wrong. It never was the battery neg earth connection.
I swapped my glowplugs control and relay for a tested one so it is not the cause.
Ii looks more like it is due to a faulty relay (probably the warning lights one).
I am gonna do a thorough check of all relays and replace the bugger one.
For the time being I'll have to try to park on the other side of the vehicle because I believe when the road is bent and the van lean on the left, the rain water drips in and humidity accumulates, causing the relay to malfunction.
The van starts fine after a while (me doing long parasitic test, checking and wiping the relays and looking for the fault every possible where), hence that relay must be drying and when it's dry, everything is fine.
The video below is (imho) the best and most complete one on the topic on the tube.
Thanks to Ratchets and wrenches.
[UPDATE]
It seems I have found the culprit.
I forgot that it's good practice to do as much diagnosing from the cabin.
I should have started there, with all doors shut.
I noticed that the driver's side indicator flashlight was flashing faster than the passenger's side.
The sound was similar to the one I heard from the relay.
I remember that touching the light bulb fitting once made something react when I was trying to figure out what was going wrong from the bonnet side.
So it clicked. I mean it clicked in my head.
and the friend who was my passenger at the time said it:
"you have a dead bulb".
I checked and he was correct.
It wasnt the earth, it wasn't the rain.
It was a defectuous old bulb that caused the circuit to disfunction.
It's replace with a new one now.
Let's see if anything else comes up!
