Here’s a translation into English of instructions that came with the Bella wiring harness from www.kla-mot-te.de
Kla-mot-te Zündapp Bella Wiring Harness Manual
The Kla-mot-te harness is more elaborate than the original harness; it has more grounding capability and concentrates wiring in the lamp unit. The original harness can be re-created using this overview: