I could be wrong, but I think that all dimming is done thru the CAN bus, including the HVAC controls, in which case none of the wires going to it is for dimming.

 

Ah, that's disappointing if that's the case.

 

The panel lamp driver circuit from the cluster (orange/grey wire) supplies power to the cup holder backlight, gear selector light, etc.

Here's another post on it: Dimmer light wire

 

Thanks, so I can t-tap into that orange/gray wire and tie that into the illumination wire on my HU?

 

Apparently it's worked for others, though I can't guarantee it would work the same for you.

 

mr_et2 was correct that the HVAC controls are managed by CANBUS, but the orange/gray illumination wire is accessible a bit further up the chain. One t-tap later and my head unit has illumination detection. Looks like the HU only detects lights on/lights off, but that's good enough.