sergod

my friend noticed last night that my right brake light wasnt working, so today i went and bought new bubls and when i installed it, it still wanst working. so i checked the fuse and it was ok. then i put the bulb that i thought wasnt working on the left side and it worked. since i have an after market rear lamp which included the wire assembly, i tried switching the wire assembly to rule out any problems w/ the sentras oem wirrings. so after switching the wires, the left side was the one that didn't work now and the right one worked just fine. now i know the problem is with the wire assembly but, why was the damn thing still not lighting when i step on the brake cause the parkinglihts are fine and the signal lights are fine. so i decided to just switch that small black box that is part of the after market rear light wire assembly and when i switced it, the right side didn't work again. now i know definitely what the damn problem is...what do you call that small black box, possibly fuses inside it? and i cant open that small box unless i brake it, seems to be one solid box except for the rubbery bottom were the wires come out......any comments, suggestions will be greatly appreciated......and sorry its long and not straight to the point.......thanks...........