2003 chevy blazer. my auxiliary light will not turn off and doors will not lock when shifted into drive

I'm assuming by auxiliary light you are referring to the interior light? Your problem could be a door jamb switch. The switch could be bad and not making contact that will send the open or closed circuit to the body control module. If the control module doesn't "see" the switch, then it will not know to turn lights on and off, lock the doors after a default time, etc.. because the body control module doesn't know the door is closed due to the bad switch.

2003 Chevy Blazing SL 4WD., I have an electrical issue, Killing my 2nd New replacement battery. On the battery, I put a test light between the the Neg Post and cable, was very bright. Pulled every fuse in the cab and under the hood. In the cab fuse box I pulled the RDO fuse, light demd down left it out. Continued and didn't change. Under the hood fuse box pull all fuses and relays. Got to the t.b.c Pulled it, tailgate now won't unlock. Test light build element shows a little red. I pulled the headlight 50 amp fuse, Now the test light shows nothing. What could be causing so many electrical problems? What the abbreviation of tbc?