In post #9 in this thread I posted a wiring diagram for the Carrera lighting.
http://forums.pelicanparts.com/porsche-911-technical-forum/743460-yea-another-electrical.html Ian found his problem was degraded wires.
In the diagram the headlights and brake lights are independent circuits. You don't mention whether the rear running lights work with the headlights on, since the ground is common for the brake and running lights. One thing you can do is step on the brake pedal and check the black/yellow wire to chassis with a meter and see if there is voltage. If you have voltage, then check for voltage to the brown ground wire. If the first check shows voltage and the second doesn't then you need to check the brown wire connection to the chassis.