First you need to isolate the 12V power source and load with inductors. This keeps the rf modulating carrier from being loaded out by the filter caps.
On the transmitting end, you need to inject the rf after the series inductor right before it is sent to the load unit.
At the load side, you need to capture and decode the rf before the series inductor.
What frequency to use and the limitations really depend on what country you will be operating in. The long 12V power wires act like antennas and will tend to radiate your rf modulation signal. You can easily emit too much signal and find the regulatory agencies on your doorstep.