I'm a little confused by what you wrote. I assume when you say 'flashing lights' you don't mean flashers but rather controlled lights that go on and off under inserts on the playfield? Also, I assume what you mean by 'non-flashing' lights are the GI lights...the ones that are always on for general illumination?

The GI light connectors are notorious for for failing on the power supply board. If you see any tarnished pins, burned connectors or housings, chances are the GI strings are either open (off) or tarnished (dim). The lamps in the backglass are on a GI string as well. Typically 2 or more for the playfield and 2 or more for the backbox. Sometimes traces on the power supply board can be burned and fuses blown that will also cause GI strings to not illuminate.

Which power supply is in your bk? The GI connectors suck butt. There is a gi relay to in black knight. Check there.

+1 for the GI relay, these can often be flaky.

+2 for GI Relay, make sure the brown and red wire are still soldered to the diode, when I bought
my BK recently and got it home the brown wire had come unsoldered.

Hopefully Hans will chime in. He's the BK expert in these necks.

Will that williams system cycle the relay guys where the owner can actually see the a/c voltage toggling on and off I know that was how I checked my sys9 GI lighting when it was out it ended up being like three bad connections lol...