2002 Mercury Grand Marquis. Backup lights won't work

I had the same problem and solved it. Follow the wire harness from the trunk lock located on the trunk lid to the trunk lid hinge torsion bar located on the Driver's side. You will see a electrical connector of wires (male to female snap in black plastic connection). I found a black wire had broken off with a clean cut. This was caused by opening and closing the trunk throughout the years which would bend that wire back and forth until it broke off as a cut. As a result since the reverse lights, trunk lights, rear license plate light and power trunk lock share the same switch integrated into the trunk lock, the open circuit(cut black wire) allows voltage to travel into the truck lock when you shift into reverse. Closing the loop or repair the wire will prevent that. After the repair, I used a strong tape to prevent the lower part of the wire harness to the black connector from moving when opening and closing the trunk.

Does pushing the trunk release button make the back up lights illuminate? Has the car been serviced or wrecked before this problem? Have you been golfing and put clubs in the huge trunk. Either the ground wire for the trunk lid devices is bad or the trunk release wire (purple/orange) and the reverse lamps wire (purple/white) have been reversed.