Hi all,
Here is a straightforward scheme for Swiss lighting.



It is quite simple to understand: starting at the left lower corner, when the grey wire is switched to (-) by the decoder, current will flow from orange (+) through all 3 leds at the front, but also through the single right led at the rear, via "y" and "z".
If one would omit the diode at the right side, current would now also flow through the rear top and left led, down via 'x' and then upward and via "z", to the grey wire. This explains the function of the diode.


The scheme posted by Gregor earlier is a dedicated scheme for the exceptional event of using double traction with 2 Swiss locs.
Basically, in that scheme, only the rear lights (L2 and L3) are switched by the grey and yellow wires, and, as usual, they may be switched off by F0.
The front lights have to be switched on and off manually, by F1 (front) or F2 (rear).
As you may see, the red & brown F1 wire draws current through both the right light (L2) and through L1, which represents the top and left light. (Older Swiss Mä locs have a single light bulb for the top and left light, and a separate one for the right light).
Thus, the front lights have to be switched on and off manually while changing direction. However, the advantage is that all lighting combinations for double traction may be made.

The diodes connected to the grey and yellow wires prevent other lights to go on (as described for the simple scheme above). To me, it appears that the diodes near L1 and L4 are not required; they probably have been added to cause a similar voltage reduction for all light bulbs, so that the brightness does not differ.