Data over power line in a car
I would not try to add signals on car power since it can be very noisy.

Your problem is exactly what CAN was invented for.  You run power and CAN to everything, then the rest is done by sending messages.

Silicon chips are cheap and light but copper wire is expensive and heavy.  The car industry decided long ago it made more sense to run a single power wire, for example, to a tailgate assembly and send messages to tell the various lights to turn themselves on and off than to run separate switched power lines to each fillament.

CAN is differential and therefore has good immunity to external noise.  For short distances like a car, it can do 1 Mbit/s.  CAN has been so common for so long that there are plenty of microcontrollers with CAN capability built in.  You will find lots of info out there with the search string "CAN bus".