Yes you are correct, in a plastic box
mod it Needs a -neg wire to finish the electrical current, but in a metal tube
mod the -neg travels through the tube, I have put threading tape on my bottom cap (on all my PVs) the same tape that you use for PVC pipes, anyways on one of my PVs it had cut the contact between the end cap and the tube and my PV mod didn't work till I took some off so the contact was present again. I have never seen a tube mod on the market that has a -neg wire up the side in order to make contact to the atty negative connection.