I thought I'd post this as a topic, instead of buried in another thread.
After playing around with my 3100 today, I finally got the reverse camera working, and it did NOT require the reverse lead on the HU. I simply wired up the video in, and powered the camera. If you go to the system menu you can set up the camera input (I don't know the difference between the battery and ground mode), and on the main source menu there is a button for Rev Cam in the upper right corner, next to ESC. Click that button, and the camera should click on. If you touch the screen, you will return to the active source, however after about 7 seconds the camera image will return. To disable the camera, go back to the main menu and click the Rev Cam again.
Doing it this way will not tell the HU to flip to the camera when you shift into reverse, but it makes the install all that much easier. Plus, you can always try to track the wire down later.
Enjoy!
