Norge brake and tail lamp circuits.

[canuguzzi]

JFYI, both the tail lamp and brake lamp circuits are hot all the time, the voltage to the brake lamp leds is just increased from 6 volts to 12 when the brakes are activated.

Something to remember when wiring anything back there. If you tie into the brake lamp circuit with anything using something other than an LED, it might not seem to create a draw but it does. Some bulb types might not illuminate or seem to but they are trying, just not enough voltage.  Basically you'd have a 12 volt bulb on a 6 volt circuit until the brake was activated as which time the voltage is increased to 12 and it would illuminate.

I know most lights we might use are LED but this is why most LEDs will illuminate off the brake light circuit even with no brake activation unless their threshold for illumination is 12 volts.

[Wayne Orwig]

This has been known discussed for a long while. If the search feature was working you could find more details. You need to cut the wire and add a diode if you add external brake lights.

[Wayne Orwig]

I don't have a 'quickie' schematic maker. But think of it as a single bulb (LED or incandescent, does not matter).
The brake light feed directly to the bulb. The tail light feed a resistor (a dimmer of sorts), which feeds a lower wattage/voltage, to the bulb. Add another brake light to the circuit, and you load down the 'dimmer resistor' and your taillight dims or goes out.

A simply diode in the brake light line fixes it. A schottky diode gives a little less voltage loss.

[atavar]

I had a thread earlier where I posted a parts list and schematic for an adapter you could put inline without cutting any wires on the bike. 
Wayne pointed out that the resistor values probably need adjusting.
http://wildguzzi.com/forum/index.php?topic=76290.msg1200209#msg1200209