I've had the same problem with my moby and found a lot of posts about this exact situation without some sort of solution.
I noticed after wiring exactly like this diagram , the entire light circuit was essentially turned into the brake/horn circuit. I figured this was because on the original magneto, there was a dedicated coil for each type of lighting circuit, and on the Treats le partie they now share the same one.
So what I ended up doing was taking out the brown wire from the split, and directed the ground from the headlight back into the brake/horn circuit via the purple wire, kind of wiring the circuit in series, which worked perfectly (my horn was out, so I bypassed that from the kill switch and horn itself). I'd assume if your stock switch & horn works you could direct the ground from the headlight back to the brown wire.
I don't know if I went about it right way, but hopefully it could work for others with the same problem.(edited)