There are 2 ways a brakelight circuit can operate, and that depends on if the brake light switches are normally open or normally closed. I believe Puchs came with normally open brake light switches, meaning one wire goes into the brake light switch from the stator (providing power) and the other wire goes out of the brake light switch to the brake light bulb. When you hit the switch, power is provided to light the brake light.
On some stators however, the brake light is off of the stator winding for the ignition - one end of the coil goes to the ignition coil, the other end is the wire for your brake light. If your stator looks like this, you need to use normally closed brake light switches where the aforementioned wire plugs into the brake light circuit AND into the brake light switches. The brake light switches need to be wired in series since pressing either one needs to interrupt the grounding out of the brake light circuit. When either brake is applied, the stator winding has no choice but to ground through the brake light bulb, thus illuminating it. This is why on many Italian bikes and Derbis and some others, the bike will die when you hit the brake with a burnt out brake light bulb. I drew a really shitty quick sketch in paint of this so you can maybe visualize it better than with my shitty words