You would think this would be easy, but I just bought my first Caddy and had to have a buddy show me how to change the tag light.
1. Remove (2) screws holding license plate.
2. Remove (4) screws under license plate.
3. Open trunk and remove front of trunk lid carpeting.
4. Remove (2) screws behind back up lights.
5. Pull out back up light/license plate holder housing from trunk.
6. Replace bulb.
7. Put everything back in reverse order.

Hopefully this helps someone.