If brake light works & tail light doesn't that socket's got a good ground - but that's all it tells us. Could be something as simple as a bulb (swap with the one on the other side) or a fuse.
By "entire taillight" do you mean the housing, or housing + socket(s)? New bulbs?
On cars with separate left & right light circuits, the dash lights are often on the opposite side to tag lights; also front & rear lights are often wired diagonally. Don't know about KJ's though & I struggle with American wiring diagrams, but that's just me & others may be able to advise which is which.