This 2001 Cadillac Deville came in with multiple complaints with the rear lighting. I will try to describe as much as possible. With the trunk open the license plate and trunk lights would glow. If the park lights were turned on the two previously mentioned lights would go out.
If you click on the picture below you should be able to see the trunk light glowing slightly.
With the park lights turned on the outer vertical strip came on as it should but the top half of the brake light would also come on. Also when the park lights are switched on the remote trunk release would not work.
After looking through wiring diagrams on the different circuits, I decided that the trunk light was probably the easiest to diagnose as its circuits are not controlled by the PZM. Since the bulb would glow, I knew there was power of some sort at the light. I pulled the bulb out and checked to see which terminal at the light assembly had power on it. I then installed the bulb and connected a jumper wire to the opposite terminal. The other end of the jumper wire was attached to a good body ground. The light worked fine with the grounded jumper wire in place.
I turned on the park lights and they were now working as well. The remote trunk also worked with the park lights on.
A little more checking and I found the broken ground wire at the left trunk hinge harness. There were two broken wires and the red heat shrink told me some one had been there before.
I also found debris in the bottom of the trunk corners that indicated to me that I was at least the third person to fix this problem. My thinking was that the other two repairs probably wound up shortening the wire length, so I wanted to extend the wiring. The most important thing to remember when repairing broken wires in a flex joint like this or a door jamb, is to never make splice where the wires will bend. In the picture below you will see that I went up the hinge arm and made my splices where the wires will be fixed to the metal arm.
On the other end, I went almost to the seat back to start my first splice. Notice also I staggered the joints, so that I would not wind up with a two inch knot of splices.
I had to tape the harness. Secure it to the trunk arm and then place the protective cover back into place.
The finished harness repair. Now, I had to install the interior panels.
This one is fixed.