As it turns out, at some point since your MY19 things certainly have changed, with respect to that light. I had occasion today to remove the washtub, er, underseat storage compartment (in the course of doing some electrical farkles). Here's a screen grab from the shop manual:
View attachment 4406
The reason I mention this is because on my MY23 C 400 GT, that luggage-compartment-light disconnection was not necessary. If you look carefully at the photo I put up in post #2, all the way over on the lower left of that pic you can see the wire that powers the underseat light ... which, as you noted, is now located on the seat platform itself, not on the side of the washtub. (So it wasn't necessary for me to disconnect anything for the light, but I did have to slide down that cable holder gizmo.)
I did this same removal about a year and a half ago on my MY22, and I don't remember which location that used, or whether I had to disconnect a switch. (And I bought the shop manual around that time, so it certainly wouldn't cover any changes made to, say, MY23 bikes.)
In any case, then, yep, there's some kind of connector you may want to check, to see if it's, uh, connected. Looks like it's over on the left (rear-brake) side of the bike.
Oh, and as always, be careful if you take out the washtub even a little bit, not to bang (or break) the little switch in the back of it, the one that lets the bike know whether the flex case is deployed or not. I did that on my first C 400 GT, and regretted it.