This 1989 Cadillac Deville came in with the park / tail lights staying on. This is a fairly common problem on 1980’s and early 1990’s Cadillacs with Twilight Sentinel headlight systems.
The Twilight sentinel module is located on the rear of the dash panel behind the headlight switch. The following picture is taken from the floor board looking up. You can see the parking brake assembly with vacuum pull off to the right of the picture and the module is the white box at the bottom left of the picture.
There are two 7mm headed screws that attach the module to the dash and the are easy to access with an extension.
After removing the module from the dash the next step is to remove the harness connector and check light operation.
The lights now worked properly with the manual switch.
I opened the module cover.
Removed the circuit board and inspected for damage. The outer bar or armature showed signs of overheating and the plastic had melted, cooled and subsequently stuck the contacts in the on position.
The customer did not want to replace the module so the wiring harness was taped up and secured out of harms way. The headlights and tail lights will work with the manual switch with no adverse effects.