[b3lha]

One final test you could do:

As far as I can tell, the time control unit is not a single circuit. It's several completely separate circuits that just happen to be inside the same box. That's why the other functions of the time control unit still work. Only the circuit that provides the duty cycle to dim the dash lights has failed.

The dimmer duty-cycle circuit , inside the time control unit, requires power to do its thing. There are four power connections on the time control unit: a3 (red), a7(blue/white), b2(blue/red) and b9(red/white) [a=B91, b=B90]. I don't know which of these is the right one, but it could be worthwhile to check that they are all live. Just set your meter to volts, connect the black probe to ground and the red probe to each one in turn.