If I remember right the GI lights are connected to a triac on the bottom of the cabinet. I know it controls the GI on the playfield. I don't remember if the triac controls the GI in the backbox. But if the bottom of it is dirty and not making good contact to the ground plate it will keep the lights from working. take that off and clean the bottom and where it screws on to make sure its making good contact. But if the GI on the playfield are working ok then its probably not that.