i suck at getting lights to work but how hard can it be? especially on a Solex, i had the same problem attacked it with vengence and still couldn't get it working.
The problem is not with the magneto since your headlight works, powers up the rear light as well. So its with the wiring from front to back. I found a pinch on the rear fender wires near the seat post right where it tucks, and a crack in the wire where it wraps around the front tube, still no luck on my end. It has to be cleanly grounded as well, think theres a short wire in the back light.