There was no relay involved for the reversing camera although it might be safer to utilize one.

I used to have a pin out for the cable NAV-TV module cable. I believe if you have a look at the "old" Yahoo NAV forum you may find some diagrams there.

I guess that's why I purchased the custom loom with video in/out, audio in and backup camera connections for the E-46 many years ago. I guess for the monies that I spent on the video TV module I didn't want to accidently short it out and took no chances. The loom looks pretty OEM with the cloth like coverings. In this entire mix I had also added the USB IBUS interface. The interface itself would use the reversing voltage (ground) to switch the video on/off in AUX mode. So in summary my AUX audio mode on my HU switches the video from BMW to aux video at the same time using the reverse camera inputs but using the IBUS instead of the backup lighting/switch. On the E46 if I go to the HU menu / TV menu I have a choice of aux video. This is one video input lead of the loom. Doing the BU camera switch picks the second video input on the loom and displays on the aux video input over riding the BMW video screen.

BTW - I made the mistake of not installing a BU camera while I had the whole rear section of the boot apart. I needed to dismantle the boot to be able to install the rear LED tails. It was somewhat time consuming.

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E83 - Aero PKG Mods installed: Homelink mirror. CIS-IBUS video module, AUX, Rr Dr Release, stock wheels, Remote start, CarPC (Atom) & mud flaps - NAV - MKIV - NA Map DVD - 2011
E46 - 18.5 wheels/tires (summer), 17 stock (winter), Homelink, NAV software upgrade, BMW video module, Radio to V53, AUX, USB IBUS interface,BMW alarm install, CarPC(Epia), remote starter, tank battery & angel eyes. NAV - MKIII - NA Map CD - High Version 2008.1