the resistor doesn't draw nearly 20 watts. In reality, the maximal draw is less than 5 watts, so a 10 watt resistor is more than enough; 20 watt is overkill really. We specified 20 watts because I had them on hand and that's what I used, and resistors of this wattage usually come in mostly weatherproof packages that are capable of withstanding the harsh underhood environment.
LEDs are a good idea and they look cool. At the time though there weren't many sidemarker options, and we thought using readily available, off-the-shelf parts would benefit more people. Incidentally the schematic in the FAQ has not been tested with aftermarket LED sidemarkers. One, LEDs are discrete devices so there is 'no half-bright'. Two, LEDs require external resistors which the schematic does not account for. Check
this thread on door LEDs for a quick primer on LED usage.
