From all you have mentioned, it sounds more like a loose connection to me. This is because it was going out over time, then failed, then worked again after the dealer worked on it. So I am thinking that the console is OK and that the bad connection needs to be found a repaired.
It is hard to say what they touched or perhaps wiggled to make it work. There is not really any "reset" procedure that is done for the DIC, except pressing and holding both buttons when certain things are displayed (like mileage & fuel used). But if the right inputs are there, I believe the DIC should work.
The schematics I sent just show the common signal that is used to set the brightness of the display - at least when the lights are turned on. And you mentioned that the radio works, but the display is not showing. I am wondering if the same thing is happening on the DIC.
Maybe you can try an experiment - that is turn the key on so that the mileage shows on the odometer, and then press the E/M button on the console. I believe if the console is working, then the mileage should change from miles to kilometers, and then back again when you press that button again.
Please let me know what you find.