I can't remember the resistance value of the resistor. About 10W though.

Basically I calculated the current flow with a normal bulb using I=P/V (since the wattage of the bulb is known). Then I calculated the current flow through the LED bulb using the same equation.

The difference between the two current values is what had to be made up, the LED bulb draws less current than the car is expecting so the resistor has to be wired in parallel with the bulb to bring it back up. A couple of sums calculating parallel resistance and that was it. Theres a handy calculator for that here http://www.1728.com/resistrs.htm. The resistance of the LED bulb can be calculated using ohms law once you have used the I=P/V equation.

Basically all you're doing is increasing the current flow through the circuit back to what it would be with a standard incandescent bulb.

If I remember I'll open the back of the light tomorrow and read it off. The LEDs were bought from Aceparts on Ebay. They look very nice as you can see. More directional, but the colour is good and they're very bright.