According to the schematic diagrams, the only other dimming function thst exists besides the headlght/dimmer switch is what I had mentioned previously: the class 2 data communication dimming function from the BCM. It goes to the radio and instrument panel.
The schmatics also show that in order for headlight/ dimmer switch variable radio & panel dimming to take place , the outside lights must be on ( regardess of control ) . This enables the 12 volt supply voltage to the headlight/dimmmer switch necessary for it to function. In your case, since you have established that the dimming takes place with the outside lights off without being able to vary the dimming further, I believe you can safely rule out the headlight/dimmer switch altogether.