Hugoneus, not to burst your bubble, but you dont need a special circuit for this.. You just need to replace the socket/bulb with a 2 stage socket. Then tap that socket into both the rear tail lights and rear brake light power wires. member named MR_Z did it a long time ago, he posted info about it in the big foglight thread in the DIY section.
Something like this:
T20 Socket for T20 Wedge 7443 Socket - Other LED Lights - LEDLight
they sell them at auto parts stores and at autolumination.com too