May not be relevant anymore, but use Ohm's law to calculate the resistance. The bulb's wattage tells you. Let's say for argument that it is a 25 watt bulb. Watts = Power = Volts x amps. So since we can assume the volts to be constant at 12.5 volts, then 25 watts / 12.5 volts = 2 amps.

Since Volts = Amps * Resistance (ohms), then Ohms = 12.5 Volts / 2 amps. This equals 6.5 ohms.

OK, that is quite a bit of resistance and heat to put into one small resistor. What I would do is put a bunch of resistors in series to add up to 6.5 ohms, thus lowering the heat flux through each resistor. Total dissapated heat is still going to be 25 watts, but the surface temperature of the resistors in series will be lower. I'd make a cable of resistors in a series loop, say about two feet long and well wrapped, and then stuff it up in the trunk overhead where the wires from the 3rd brake light come down and the vacuum lines for the rear headrests are.

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