OK. Baby steps. Before you can learn to fix anything you first have to know what it is.
Here is a reasonable copy of your manual. Print it out if you can. Sometimes there is no replacement for hard copy.
A brief entry primer. The Stern you bought it not hard to work on. But you are going to need tools. As well as the correct screwdrivers you will need pliers. You will need a soldering iron and solder and you will need to learn how to solder.
And you need to know what the parts are or there is no way anyone can communicate with you. And there is no way you can communicate with anyone.
You have mechanical parts. You have electronic parts. Learn the manual.
You have 5 circuit boards in Galaxy. Underneath the play field in front of the transformer you have what is commonly called rectifier board--less commonly refered to as A-2 You have 3 connectors on A-2. Upper left is connector J-2, upper right is J-2, and the large connector is J-3. J-1 controls your GI lights (red wire), feature/play field lights (blue wire), ground wire for GI lights (white wire). ----more to come when I get home next week and have my notes in front of me.
Inside your back box you have 3 more large circuit boards. Upper left is the MPU which is the brains of your pin. Upper right is the SDU (solenoid driver unit). The SDU is loaded up with lots of transistors that control all of your solenoid coils that work the flippers, pops, slingshots, etc.
Lower left is the lamp driver board that makes ALL of your play field lights burn.
On the upper left side is a smaller board that is your sound board.
These 5 boards are A1,A2,A3, A4 and A5.
Here is another brief primer until I get home. This is electronics/schematic talk. I am going to tell you to take a close look at A2-J1-2. What I just said was to look at the red wire on the rectifier board A2. Look at the connector on J1, the upper left connector on A2. This connector can hold up to eight wires numbered 1 thru eight from left to right. You look at wire #2. If my memory serves me correctly, A2-J1-2 is the red wire that power your GI lights. And I believe A2-J1-6 is the white GI ground wire.
I'm done now. Read the manual. Learn some stuff. Try to come with lots of questions and write them down. I'll help you all I can when I get home. And it looks like you already have quite a few standing by to help out.