Something is amiss with electricity getting into the metal of the tube causing the tube to glow in the dark.
Check for frayed wires contacting the tube. The suggestion about checking for a good cab ground is appropriate. Somehow, the dash lights and such in the cab may be returning to ground through the tube.
The funny part about electrons is that they will find a path, any path, even ones they are not supposed to use, and in the absence of a path, they will leap across gaps. Of course, in the case of spark plugs, this is good.
I apologize for sounding so insultingly simplistic. I don't mean to be talking down to anyone.
