Just used the wiring description in this thread to locate the backup wire that goes by the emergency brake down under the driver footwell. Found out there are two black wires with a pink stripe...
. The first when probed was 6v with the ignition on, and didn't change in reverse....Second one was correct (2006 E350).
I ended going with a separate 7" Amazon (Dragonpad) monitor and Esky camera. They work pretty well together although the camera would have been better placed up high as it is advertised as 170deg, but really is more like 130deg, so very close things on the side disappear. I had a similar backup system on my earlier van, and the camera was definitely wider angle and much better for that last few inches of backing. This one may require some adjusting..
The descriptions here were a definite help!