The switched lights or the GI? the MPU isnt all that relevant.. the backbox lighting comes from the power supply.

look at the test points and pins on the power supply in the cabinet, look at the power supply schematic.

Tool to help debug the GI (or any lamps)...solder two clips onto the appropriate areas of a spare lamp socket, install bulb, clip where needed in order to test.

Bob

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.