If all of the cpu back box lights are out and don't work in test mode, start by checking the connector and also the bundle where it goes into the box for any pinched wires.

Good with dmm? You can test for voltage at the lights with the machine on. That will help tell you if it's a power side problem or a ground side issue.

Do you have the schematics?

If not, you can do a manual trace to see which wires go from the bulbs to the board. And the connectors to test.

If you're comfortable checking things with power on, turn on the machine and go into the light test in adjustments. Once there and they are flashing check for power between the sockets and cabinet ground. Should get 6v (I believe ) cycling on and off. If you do then we know the power side is ok

continue to the next step

With the power off, disconnect the connecter at the board and with the DMM check for continuity from each socket to its respective wire in the connecter. This will verify all of the controlled ground wires to the board are good.

If both of these check out then it's a board issue.

Imo, if it's only the back box controlled lights that are out its likely a voltage supply problem.

I would be too.

6v at the sockets means you are good on the power side.

if you have continuity on the ground side from the sockets to the board/connection then I can't see it being anything else then a board issue.

If you can get schematics that would be great.