I would try using a power feed from the red wire to the horn (Tromba). From what I read on the Google group, the stop light bulb is not earthed in the tail light body, the circuit is completed when the switch closes and makes contact to the chassis.
Don’t use a twin filament stop tail lamp.
Hi Don, I don’t think it is not that simple. The brake switch is normally closed. One terminal is earthed, so, by implication any power taken to the other terminal results in a short. Also when the brake is operated the switch opens and therefore no current will flow to the brake light bulb. Your idea would work if the switch was a normally open type which closes when the brake pedal is operated. Why Moto Guzzi did not design it like that in the first place is open for debate. I think it is to do with the very low power output of the lighting coil. It only produces 28 watts which only just covers the headlight and tail light. My thought now is that with such a marginal power output from the lighting coil they decided to use the ignition coil as the power source for the brake light. My challenge is how to electrically isolate the ignition coil so that I can run its earth through the brake light switch. I am wondering if when the brake light law was introduced in Italy, the simplest solution was to install the switch (which does indeed look very much an afterthought), snip the coil wire which is soldered to the coil support and solder it to a wire which was then run to the switch. There is a strange reference in the owners manual in which it says if the motor is stopped whilst the brake switch is operated the bulb will blow, if this occurs the bulb must to be replaced. Assuming my translation is correct it makes sense because if the bulb is blown and the brake pedal operated the spark will cease as there is no earth on the coil. come on chaps, there must be someone out there who has a fully running Cardellino 73, with original brake switch, who can put me out of my misery!!! Cheers
Phil
Aha! Problem solved! The earth wire on the ignition coil (The copper wire soldered to coil support on the left hand side of the photo) was unsoldered and then soldered to a length of black wire (number three on the wiring diagram). A 30mm length of of shrink sleeve was then slipped over the soldered section to ensure it did not earth out on anything. The other end of the No3 black wire was connected to the brake switch. Hey presto the bike started and when the brake was operated the brake light illuminated!!
Cheers Phil
Well done Phil 
Congratulations. That certainly was a challenge to solve 