Thanks again ppro. Tonight I got out the trouble light and a magnifying glass and had another look. The wire I thought was solid grey, was not. It is gray with a mauve stripe. So the 4 gray wires are gray with mauve stripe, gray with black stripe, gray with red stripe and gray with green stripe. So for the back-up signal, the instructions for my car do not apply. I did find the gray with yellow stripe wire coming from my passenger side back-up light socket just as ppro had suggested. I'm going to follow it as far as I can towards the front of the car and splice into it. Hopefully, I won't have to remove too much of the interior trim.