Straight forward.
Run the rca back to the head unit.
I also run a trace wire back as well.
I tap into reverse via a trailer harness or off the back of the tail light harness. I never tap into the main harness. I always attach a single pole female crimped terminal and cover off the reverse wire.
I never use butt connectors. Ever.
I have a matching single pole male connector on the end of the trace wire at the rear of the car.
Tape the trace to the rca intermittently along the length.
Then at the front of the car, terminate the trace where it ends with the rca, with a single pole female connector.
You will have a reverse input wire from the head unit which will connect to the trace from the back with a male single pole connector.
Earth via either the harness, or as a new earth to the body.
I always add the connectors because never be the person who just joins it all up to get it working and leave a mess that isn’t easily disconnected for someone else.
Having the connectors in that order keeps things safe from shorting if they are ever not connected.
Where the rca is joined in the front and the rear, tape up with some tesa tape to stop rattling and keep it plugged together.
You can buy the connectors in packs by Narva at car shops and you’ll need the special crimp tool to attach them. Car shops sell it.
Again, don’t use those shitty red, blue and black butt connectors.