On the problem deck, check every pin inside the Molex connector housing where it connects to the other harness that runs back to the backplane board.
If just one ground pin is pushed in and not touching the female pin, you will not have any lights.
I'm assuming you have the 20-pin, 2-row Molex plug on the problem deck...or something else?
If I were you, I'd post some pictures of your button deck connections.
Maybe someone will spot something amiss or we'll be here for 3 weeks and 100+ Replies guessing what you have.
If you have a digital multimeter, check for continuity with every single wire on the problem deck.
There's only 20 wires....shouldn't take too long to find the bad one.
The harness with the mostly white wires, is the one you want to check...it's for the button lamps.
The other smaller harness with mostly orange wires, is for the microswitches.
I know you've said you've tried other harnesses...but there's no other answer right now.
How come the other deck worked and not this one?
Was the problem narrowed down to the deck buttons or one of the harnesses?
Swap again with the good parts, ONE by ONE, until you find out exactly what's NOT working.