Yes, the factory rack does need to be uninstalled by removing the 4 retaining screws in the top. When you get it off, there is still a ledge of plastic above the factory light assembly. That ledge of plastic is part of the entire plastic underseat structure.
Here's a shot from the rear with the factory cover removed, showing the ledge.
Then the light assembly gets removed. I'll expound on that when I make up the instructions, but here's a shot with it taken out. You can see that upper ledge is a permanent part of the rear assembly.
The light has to be removed in order to facilitate getting my light wiring into place. Like I said, I'll post up a full detail on the installation shortly.
Finally, the light is 7/8" (22.2mm) tall x 4" (102mm) wide x 1 1/8" (28.6mm) deep. It is tapered in the rear so it tucks down into place.