Cabnboy23

Hey guys! New to the MBWorld as I recently bought a 2008 ML350 for my daughter. I am in the midst of installing an Android 12 Head Unit and im not getting the signal for the factory backup camera. I have spliced into the black and white wire in the MB harness which i believe provides the signal and have it going into a male yellow rca plug going into the head unit for the camera input. Do i need to really run a wire from the rear tail light for power? Isnt there a way to tap into the OEM wires behind the HU to get this? 12v to 6v converter needed ? The HU works fine for radio, BT, Nav, so i have that all wired up i believe correctly.
What i am looking for is either the correct wires to tap into for the camera for power or someone to explain to me why i would have to run the wire from the tail light to get the camera to power on.
I did not have the green fakra (which would have been nice) so i am looking for my best options here so i can wrap this up. I can provide pictures of my work so far if that would help.
Any help is appreciated and this forum has helped alot along my journey with this new MB so far...thank you!