Sounds like one of the GI lamp sockets is shorted to ground. I had an issue with a Fathom once where all the GI lamps would dim ONLY when I closed the headbox light panel. Initially the GI fuse blew on power on. As I diagnosed the problem, the lamps were all working when the light panel was open and then immediately dim when I closed it. Open the door and back to normal. Close it and they all dim. Is was the lower common wire at the bottom of the lamp panel shorting against the earth strap of the head.

I would take the approach of splitting the problem by removing the connector for GI on the headbox. Then you know where the problem is - lower cab or headbox.