So, as the title says, when I open the doors all the lights illuminate, as they probably should, but I cannot turn them on when I'm driving for example. What could be the problem? Sometimes one of them does start working randomly...
The terms you use to describe voltage are incorrect which is perhaps making it harder for you to understand. Think of a battery. There is what we term a potential difference between positive and negative terminals. Open circuit is nothing at all. Going on that, your first picture describes a situation where the light won't work. Sometimes using electronic tools makes it harder to understand. Terminal 1 in 1st picture shows 12v +ive because the bulbs are in. With bulbs out, there would be nothing. Bulb filaments make a circuit.
From the left your terminals are as follows:
1. Switched earth (12v -ive) by switches in door latches. This is working.
2. Positive supply (12v +ive). This is working.
3. Permanent earth (12v -ive). The circuit needs this for the individual lamps to be switched on by operating them. This is where the fault is.
I do not know where the fault is exactly. However, it would be easy to take a spur from the permanent negative in wires going to the front courtesy lamp and connect it to the wire of one of the rear lamps. You could trace the wires for the rear lamps but I suspect it is not a separate earth for them. It would make more sense that there is a common earth and the earth wire to the rear lamps has become open circuit (broken wire/corroded wire/heavy oxidation at junction).
When I had that fault, I was lazy and just took a spur from a good earth. That car was not a keeper which would last so no need to do it properly.
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