If your bulbs aren't too ridiculously bright, this circuit should work okay... The TIP31 is technically rated up to 3 amps, which is around 35 watts in your car. The only modification you'd need would be a heat sink on the transistor.
You could equivalently use two or four of these circuits to put less load on the individual transistors. They're cheap and small.
If you do put a heat sink on it, be careful. The heat sink is attached to the collector, which is connected to the Illum+ line. Use electrically nonconductive thermal compound so that the heat sink doesn't short out to ground accidentally.
I can put the circuit together for you if you want; PM me.