My father purchased a 2006 RX400h with 73k miles on it. The seller said the reversing camera works only sometimes, but now it seems to have gone completely dark. When reverse gear is selected, the nav screen goes dark, and that's it. Otherwise, the nav screen and all other features work perfectly.

I read a post by someone saying theirs showed a dark screen with a text ("be wary of your surroundings" or something). I don't know whether this is a feature in European cars as well, and so whether his not showing a text like that is a clue of some kind.

What should we investigate first? The camera, the wiring between the camera and the parking assist computer, tailgate wiring to the roof, connections behind the satnav, or...?

 

Disconnecting the camera makes no difference; screen stays black with no errors shown. Sometimes, however, connecting the camera or wiggling the wires caused dancing patterns like this to appear on the screen:

 

We fitted an aftermarket camera, got 12V for it from the reversing light (OEM camera is 6V), and it works:





Those guidelines are a little unfortunate, but tolerable.

Any generic aftermarket reversing camera ought to work, but there are large differences in field of view, and for a reversing camera I would pick a very wide angle one. The one we fitted had a really wide angle.
We also had another camera prepared, which had perhaps an even wider field of view and no guidelines, but that one went on my parent's RV. If I were fitting a replacement reversing camera now, I would go with the latter one.

Since the reversing lights on the RX400h are quite poor, we also fitted these LEDs (the "24 SMD 3030 LED" option). They are very bright, even with the metal bulb shields in place (to prevent excessive glare).

EDIT:
If you power an aftermarket camera off the reverse light, the camera will not work in the diagnostics menu "camera check", because that does not trigger the reverse lights. One could probably achieve that by tapping some other 12V source with a relay, switched by the 6V power cable of the original camera.

Instructions for the diagnostics menu, just in case, here:
www.youtube.com/watch?v=4UcgYqsWB1o