serial - Data over power line in a car

I am trying to build a communications system for a car, for turning gadgets on or off. One idea was to parasite on power lines, but what kind of components and signaling could be used for this? I am thinking pretty low communication speed is acceptable, such as below a megabit, but more than 10 kilobits per second.

Is there some suitable standard for a hostile environment such as a car?

Answer

There are two commercial solutions that I'm aware of; the first is PLC4trucks (SAE J2497) which is (or at least was) based on generally difficult to obtain Intellon ICs, the second is through Yamar Electronics who offer a range of devices that would seem to meet your requirements.