I've been a little side-tracked finishing up the fog lights for the cars and working on the basement so we can rent out the house when we move, but I've managed to put together this schematic of how I intend to wire the brake light up. I used the open source TinyCad from sourceforge. Pretty cool little program.
The white LED's are because many states require illumination on the license plate, and obviously red LED's won't provide that. I don't want to attract the wrong kind of attention with this. The schottky diode is to have minimal voltage loss on my primary power source, while preventing the high current for the red LED's from kicking back into the low power white LED's.
Most of the circuit looks very much like the one from the constant current power source linked in the first post, but you'll note that I've added an additional NFET and resistor. When power is applied to the brake signal wire, current will flow through this transistor dropping a second resistor in parallel with the first. This will alter the current set point of the system, making the LED's brighten dramatically. They're rated for 700mA maximum DC current to provide 94lm of output.
10 ohm for taillight = 50mA of current through red LED's ~7.5lm each, 90lm total
.758 ohm for brake = 660mA of current through red LED's ~85lm each, 1020lm total
Reminder - in the picture above they're being driven at 150mA, and there are only 6 of them vs 12...
Changed R4 to 39 ohm, after measuring a constant 13.9 volts on taillight pin to keep those 4 LED's happy at 30mA.
Added R5 to catch any potential current spikes on application of brakes.
Added R6 to drain charge from Q3 to allow channel to close on brake release.
Added diagram on right:
Mostly just a copy from the instructable, with values customized for my application. ~650mA to 5 LED's for the turn signals.
I needed a wire connector for the switch harness on my wife's car, so had to place an order from Mouser anyway. To save on shipping costs, it only made sense to buy the parts for this at the same time... I'll post up if I find an excuse to put it together this spring before we move!