I just replaced the 3rd brake light on a 2001 DTS. Would be the same procedure for Devilles and DHS’s of the same type.
I thought I’d post this to save others some time and concern about tackling this fairly easy task.
The 3rd brake light is a complete unit that includes the lens, LED’s and circuit board.
It can be replaced by removing (5) nuts from bolts on the inside of the trunk and removing the unit from the outside of the trunk.
First step is to loosen the trunk liner. This was accomplished by carefully pulling on the lower portion of the trunk liner around the (6) plastic fasteners that hold that portion in place. That is easier than trying to extract the fasteners first. Once the liner has been pulled down over the six fasteners, you can take a pair of needle nose pliers and carefully remove the fasteners by grasping them around the shaft and pulling them out for reuse.
You will need to pull the liner down from around the emergency trunk opener pull and another metal hoop that appears to be there for tie down usage.
Now you have access to the five nuts. The ones on this car were only hand tight.
Remove the 5 nuts and unplug the 3rd brake light connector.
For removal of the unit, itself, I’d recommend the following: apply lighter fluid around as much of the brake light unit as possible to loosen the adhesive that is on the seal between the unit and the trunk. I did not do this, and it made removal a little more challenging. The unit was so tightly affixed to the trunk that I thought I’d missed a bolt. I finally used a wooden spatula (so as to not scratch the paint) and pried the unit a little at a time until it all popped loose.
I’ve used lighter fluid for a lot of adhesive removal applications, and it does not damage paint, and I’m sure it would have made the job much easier and less stressful.
You can now install the new unit by positioning it on the trunk, affixing the five nuts and connecting the connector.
Dealer allows 30 minutes of labor to do this and ours charges $250 for the entire job. The unit sells at the dealer for $165. I was lucky enough to have a “friend in the business” and got mine for $119.
Savings was well worth the time and effort. If I’d known the above, I could have done it in about a half hour myself. As it was, it probably took me closer to an hour.