Hi currieddog , callmedoor
I post my "hypothesis of functions" made from https://www.ipdb.org/showpic.pl?id=24&picno=77594 : On the bottom of the Backglass we see "Balls to Play - 1 to 10 which is the Ball-Count-Unit Positions 0 through 9", the (reverse) Text Bonus Ball is what we call "Extraball, Same Player shoots again" - in the schematics operated by "Bonus Ball Relay". Also in the ipdb-picture, reverse Backglass I see a little square for the Replay-Counter - better to be seen here https://www.ipdb.org/showpic.pl?id=24&picno=23562 to the left of the green aeroplane --- we are used to say Replay-Counter - but in the schematics "Extra-Play-Unit" is used for the Replay-Counter.
Thesis - when set in "Replay-Mode": When starting a new game the Ball-Count-Unit is stepped to (visible) 5 balls to play - as we play - one by one stepped down - BUT we can make an Extraball - shown in the Backglass, Playfield as "Bonus Ball" --- we may play good and make an Replay - the (((Replay-Counter))) Extra-Play-Unit is stepped up one step.
Thesis - when set in "Add-A-Ball-Mode": When starting a new game the Ball-Count-Unit is stepped to (visible) 5 balls to play - as we play - one by one stepped down - BUT we may play good - get "free balls" - the Ball-Count-Unit may step up, up, up to (visible in the Backglass) 1,2,3,4,5,6,7,8,9,10 Balls to play.
Thesis - callmedoor is playing in "Replay-Mode" - the "Bonus-Ball" Light in the Backglass does not function, the light here https://www.ipdb.org/showpic.pl?id=24&picno=23563 (on the bottom called Bonus-Ball) does not function - but when he makes an "Extraball is called Bonus-Ball", looses ball-in-play: THEN the Ball Count Unit does NOT step down (and thats what we want - we kind of play the same ball once more).
I do not know the game - it is a late EM --- they may have implemented what I wrote as my hypothesis. Greetings Rolf

Hi callmedoor
You gain an Bonus-Ball - the light on the playfield blinks - You loose ball in play - does the light still blinks ? (it should) - You then launch the given ball and make points - does the light turns off ? (it must).
Where is Your Bonus-Ball-Relay mounted ? On the bottom in the cabinet ? If "Yes" You have an wire running through an Jones-Plug up to the playfield-bonus-ball-light AND You have an wire running through an Jones-Plug up to the backbox-bonus-ball-light - all good with this Jones-Plug ?
(((When You have fixed the fault - may want to change the pin from "Replay-Mode" to "Extraball-Mode, Add-A-Ball-Mode" and test my theory of "ball-counter visible in the Backglass will step up" ?))) , (((it is getting late here in Switzerland - time to go to sleep))) Greetings Rolf