purdiemerc

Hi
Help Needed ASAP
I am getting a towbar fitted to My S550 W221 to tow an Adria 402 PH, and I wanted to install a backup camera on the back of the van and have that come up in the comand instead of the one on the back of the car when the caravan is attached and I engage reverse.
I intend doing this by switching the video input from the car camera, using a 12V activated switch I got on EBay, so when the switch gets 12V from the caravan via the hitch the system will get video from that source at the back of the van.

My problem is that I am having trouble identifying what wires are Shield and Video.
The connector for the OEM Camera has (Red, Black, Yellow, Black) which I would assume is 12V, Gnd, Video, Sheild but when the car is in reverse, I do not get 12V on the red !?
The after Market Camera has RCA Video and 12V Power Plug, I can simply connect it to 12V but where will I feed the video output from my switch ?
The wires on the other side of the OEM connector forom the OEM camera to the car have different colours of (Yellow, white/clear, Brown, Black) connected to (Red, Black, Yellow, Black) Respectively.
So am correct in assuming the Black and yellow from my OEM camera go to the video input to the comand or is it the yellow and white/Clear - (that looks like shild braid in a clear sheath) on the car side of the commector ?

I would realy appreciate it if someone can sort this out.
Thanks
Purdiemerc