12V active-low on/off switch
You could make a high side switch with a p-channel MOSFET. Attach your load to the drain, and your car's 12V power to the source. The MOSFET basically becomes a short when the voltage on the base goes lower than the source voltage, 12V in your case, minus the Vgs(th). If the data sheet for your MOSFET indicates that the Max Vgs is greater than 12, then you can attach the signal to the gate through 100 Ohm resistor and be done.

If you are interested in a schematic, and more details, search "MOSFET high side switch"