Can't change bulb

Did you check the fuses in your mystery vehicle?

There can only be a few reasons

Bulbs
The Brake Light Switch
Fuses
Broken Wiring and grounds in the trunk

..

A fast blinking light is indicative of a bulb out. If all the bulbs are working check them out especially the contacts in the bulb holders. If all is fine change the flasher relay. If none of this works, it may well be the wiring, and that can be a nightmare to trace.

you more then likely fried the board since led bulbs need a converter to make them work. or when you disconnected the board you broke a contact point for the wiring harness connector

I've got a 2008 cts. And I just checked the lights...None of the rear lights can be changed. NONE! I do not know where these people get their info from. The only light that can be changed are the HID, fog front turn signals and the DRL's!

Check the wiring going to the chassis holding the brake light bulbs. In the trunk when setting groceries and other items inside someone may have knocked a wire loose.
When you say none of the brake lights did you mean turn signals and parking lights as well on thefront and back?

Sounds like something is connecting to the car body, I would inspect the bulb you changed make sure the metal clips or wires are not touching anything.

if you had the unit plugged in when you changed the bulb, then you may have overloaded the circuit board and caused the tracers to burn off the board.
it happens alot when the bulbs are changed.
if you are able to do soldering, then you might be able to put a jumper wire on the board, if not you might have to replace the control board.

which light bulbs were changed? recheck all that were replaced. do the hazard lights work?

Check for screws from the inside of the trunk,  If none then unit should slide towards front of vehicle, many melt into place due to high bulb temp in small plastic housing