You need to pick up the live from the reversing light in the car rear light cluster or wiring loom nearby.
Remove the rear light and test with a meter or test lamp to see which wire is live only when the reverse light is on.
This goes to the yellow wire to pin 1 in the 12S socket
I prefer to join by stripping about 2cm of the car feed and bending the wire back twist it to form a spike.
Crimp a female bullet connector onto the spike and a male bullet onto the lead from the 12S socket.
This gives a much better area of contact then scotchlocks.