Tail lights

[Bamer36]

alright need a little help my tail lights work but don't. I know that they turn on but there's a wire that comes from the bottom of the cab into the cab that the prior owner had plugged into the fuse box which gives it power but there on all the time the turn signals work on them but when hitting the brakes they don't get brighter or dim. I know it doesn't go right into the fuse box but have no idea where it goes can't figure out the wiring diagram any help will be grand. I have a 63 SWB c10