Impala/Caprice Installation Page 13
Here's the control box in place. Connect the control wire connector together, remove excess slack from control wire (gray wire) any excess slack can be stuffed behind rear seat, and tie wrap or tape the control wire to the factory harness. The control box fits snug in the corner and will not come out on its own.
Note: The reasons I selected this place to mount the control box are: 1) You can adjust it while looking at the tail lights. 2) One of the easier spots to interface with the tail lights. 3) It becomes almost invisible when complete.
The black wire coming from the control wire is the ground wire. It follows the control wire back to the trunk hinge and connects to this grounding screw.
Here's a close up of the ground screw connection.
Now the control box is out of sight. You can turn the ignition on and test everything out - see users manual.