Schematics are available, try, https://www.flippers.com/
Since ALL backbox lights are off...
After you check the fuse clips, make sure they are not rusted, broken, missing, and the fuse is present/intact (the fuse you want is in front of the lower panel inside the coin door), you should be testing to ensure that you have +6v coming in from the transformer to that specific wire line. With the game off, you also want to validate continuity for the wiring harness with a DMM. Additional note, is you need to consider either dremeling the crap out of the bayonet sockets, or replace any socket that is corroded/falling apart.
Keep in mind, not all lights are general illumination.
Some lights such as ball count and match are feature driven based on the steppers, so those have to be working properly as well.
If they have not been rebuilt, I don't know what is going on, but these lamps will not light when the left flipper is touched, except game over, tilt (if activated), and last match number.
Long live Ted Zale and the mighty legacy of "zipper flippers".
