I have never figured out the wiring on a 6v Motobecane because the wiring diagram does not have a picture of the internal light switch connections. I do know it is strang, as the tail lamp and brake lamp fillaments are both wired to the brake lamp switches. I can only assume that the tail lamp goes out when the brake lamp is on, but I dont know this for sure.
The voltage regulator is a Zener diode, I think it is mounted on the steering hed where it is grounded and has a single black wire connected to what looks like the brake lamp circuit.
Get your hands on a vom and make sure that the tail lamp socket has a good ground, the bulb is grounded to it and both connections to the bulb base are good. Also, look closely at the bulb and determine if one or both filaments are glowing and if anything changes when you apply the brakes.
I dont think any of this affects the horn, use a vom and check for voltage and ground.