There could very well be different fuses for the different 'actions' on the back tails. I am not 100% sure but fuses are either good or bad, no inbetween so that would explain maybe a multiple fuse setup.
I am just guessing but I know that cause the bulbs are on and then can get brighter also that maybe it's like the setup for headlights. You know when the headlight burns out but the hibeam still works on some cars, maybe something like that. Either way, even if you took it in to the dealership, they can check it real quick and let ya know, might not even be a warranty issue and just a $2 bulb or something. Good luck....