With the power off, check continuity from the switch lug connected to one of the return wires to the Molex connector corresponding to that wire in the back box. If no continuity, you may have a broken wire somewhere, or maybe need to repin the connector. Check all switches in the row.
If there is continuity, then I would remove the Molex connector and (now with the power on) take a jumper wire and with the game in switch test mode, jump the send and return pins corresponding to each switch in that row. If they don’t register, then it seems like a board problem.
Note that I’m unfamiliar with this game and Gottliebs in general, but I assume they work in the same way. Someone more familiar with them can perhaps confirm.
If you are not sure how to do this, let me know, and I can provide more detail.