Testing the tail light is made easy since the connector is just inside the license tag/gas door on the back trunk wall. Looking at pictures below, just flip the gas door down, reach over and unplug the connector on the left at back trunk wall, and apply 12 volts to each of the 3 connector.
- Light Green control backup light
- Purple controls brake and turn signal
- Dark Green controls tail light/park light and license plate light.
Check each one to see if they are working if 12 volts is applied. If they all work your problem is in front of the connector. If one doesn't work, the bulb might be burned out, you might have a grounding issue, or the socket is not making good contact.
Access to the tail light bulbs requires removing the center tail lamp center ornament bar (3 sets of 2 screws), then the tall clear tail light lens via top and bottom screw and then gently prying the lens away from the gasket and removing/checking the bulbs. Sometimes the spring that pushes the contact up toward the bulb base tip gets corroded and you can work it back and forth with needle nose pliers and WD40. Sometimes they base of the fixture loses contact with the tail light fixture. You can usually take punch and notch up against the rim to make ground contact again. If all else fails there are push in spring type replacement lights bulbs in the "Help" section of most auto stores. This requires removing the tail light fixture, and prying the lip off the old bulb so that you can remove it. Hopefully you just have a burned out bulb, or the spring isn't making contact and can be fixed.
Just one other note, I have used a heater hose... I think maybe 3/4" hose to push onto the bulb to be able to push in and turn out. The back up light in particular is hard to reach in and get your hand on.
Here are the pictures of testing the tail lights from the back wall connector. I am using a simple 1amp 12 volt AC/DC adaptor with alligator clips, however you can use any 12 volt DC source.
Flip door down and see connector over the left and unplug:
Here is a close up with connector unplugged:
Observe the lights as you test. Here I have the 12 volts on the light green wire for the back up lights.
I use folded aluminum foil to fill the gap between the fixture and the bulbs base so contact is made after the WD -40 to work the spring
For any side to side movement a toothpick and a dab of crazy glue will stabilize the bulb
Prevents bumps from jiggling the bulb loose
Living in Florida for 13 years you pick up some inovative Red Neck Tricks
If you have to move your transmission to take a bath -- you could be --a Red Neck ---LOL !!!!!
Real nice extremely detailed taillight electrical repair info
A lot of variables going on there
Electrical issues can be challenging
Nailed the subject
I took some notes as i often do
I have an accordian file of all detail notes from the Cadillac websites i have collected since 2006
Just always enjoying knowing all i can about the 64 Cadillac
Clean all conections to shinny metal
The wiring going into the plug can be removed ( backed out ) from the plug by slipping a small flat tool into the socket
Removing the wire from the plug will help you access the terminal to clean to well
They sell a spray to clean plug in terminals
I always clean them by hand myself