Use 3 3.5 volt LED's in series. That's 10.5 volts of diode drop from the LED's right there, good place to start. Obviously you'd need to do them in three's so you'd need 18 LED's 3X6, or 15 for 3X5. So you would need either 5 or 6 drivers (you may be able to find a multiple driver output IC)
I hate all these car applications that use 3.5 volt LEDs and a resistor to drop the rest, the entire point of LED's is they're efficient, if you add a big series resistor to control current you're throwing the heat away yourself, might as well just use a normal incandescent bulb!