AVH-P3200DVD: can't get the rear view camera working with the reverse gear

Hi everyone, I'm working on AVH-P3200DVD with backup camera on a Hummer H2. However I can't get it working automatically with the reverse gear. Here is what I did:

1 connect the camera to a 12v source seperately from the car (I will eventually hook up the power with the car, it is just for testing now)

2 connect the camera RCA connector to the DVD unit (RC in)

3 connect the violet/white cord from the unit to the positive cord of the back-up lamp

4 set the camera polarity to "Battery" (I tried Ground also, no any differences)

So, as a result, if I push the "rear view" function on the unit, it did shows the rear view image perfectly. But I have to do this manually. It will not shift to rear view automatically when I shift to reverse gear. I check the cord from the reverse lamp and make sure there is 12v coming in when I shift to reverse. So I couldn't figure out what I did wrong, anybody has any ideas?

Thanks

Livio