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.