In principle all the wiring had been completed, but two things needed to be done: the internal lighting of the switch for the external 12V supply needed to be connected, and the ampmeter for the internal consumers needed to be properly wired so that it also would show current when the power supply was used instead of the solar/leisure battery circuits.
The switch does connect the POWER-ON line with the GROUND from the power supply. I decided to use the -12V as the other connector for the switch illumination, so that the direction of the current is the same as originally intended in this switch (there is ACC, power, and ground connection at the switch, and since ACC and power are already on ground level, the remaining connection should not be +12V; this is important if the illuminator is an LED, where the direction of the current matters).
The ampmeter just needed some rewiring of the common ground return cable, which I had connected to the common terminal block instead of going through the ampmeter resistor first. I changed this, and now the meter also shows the current when the consumers in the car are connected to the 12V power supply instead of the leisure battery.
The cabling looks chaotic, but is all well-thought-through :) :
Once the cabling is hidden and the big wiring board is moved and affixed into its position, it looks more tidy. Even the coffee machine is back in place: