Try a 10 ohm 5W resistor - see how that works.
But for a start I would make a rough (large) version, and test how it works, then worry about making it more compact once you've proved it.
As it's only for brake lights, how about replacing the brake light with LED's? (as is becoming common now), and using the alternator to charge a small battery to power just the brake light?, with a constant current driver for the LED's. This would ensure the brake light works well at all speeds.