Here ya go:
1. Start with the engine off, close the door,
wait 30+ seconds until all dash lights,
interior lights, and exterior lights are off.
2. Press and hold OK Button on the steering wheel and don't release it yet.
3. Push Start Button only (no brake pedal) to
turn on Accessory Mode.
4. Once the message "Full Accessory Power Active" appears, release the OK button, then quickly press the brake pedal and turn the engine on.
5. Engineering Test Mode appears instantly.
I tried this method and couldn't get it to work for me at all, and I tried a few times. If anyone was walking by I'm sure I looked like an idiot starting and stopping my vehicle in my garage LOL.
Here is what worked for me and the vehicle did not need to be running either (but it can be running, I tested both ways):
1. Start with the engine off, close the door, wait 30+ seconds until all dash lights, interior lights, and exterior lights are off.
2. Press and hold 'OK' Button on the steering wheel (right side) and don't release it yet.
3. Push Start Button only (no brake pedal) to turn on Accessory Mode, continue to hold 'OK' until Engineering Test Mode is displayed on the right of the cluster.
4. Once done hit the "Back" button to exit or shut the vehicle off.