24 volt LEDS will either not work at all on 12 volts or they will be very dim. Supplying less than the rated voltage of anything will always result in diminished performance. The dual intensity LEDs should have a diode and resistor in series with the taillight wire that reduces the current to all the LEDs and the brake light wire bypasses the resistor to give full current to the LEDs, but I haven't seen a schematic of the arrays to see if that is how they did it. If half the LEDs were dedicated to each function, the brake lights would not be bright enough in the daylight.
