I’ve now looked at a Sprinter wiring diagram and physically checked. The flasher unit is an integral part of the steering column fuse box. No way of getting a feed there alas. So the side camera feed would have to come after the flasher which means I need a timer. 5 seconds or so sounds about right. The always on rear camera is powered by the ignition as is the Pioneer head unit with the monitor. Once they are on the image goes from the camera to monitor without any trigger being required( as opposed to the reversing camera which needs a trigger from reversing light to bring the image onto the monitor). So my thinking ( thank you Lenny HB) was to have the video feeds from the always on camera and the side camera go through a 5 pin changeover relay. The trigger for the change to the side camera video line would be the 12v output from the timer circuit en route to the side camera. So there should not be more than one of the cameras on at the same time. Alternatively being the rca lines from the always on and side cameras into a Y adapter before the headunit. Then power from the timer for the side camera and power for the always on camera through a 5 pin relay so that agin only one camera is on at the same time ( to stop cross over of images).
Does all this sound too complicated ?!! If not then thoughts please on this and the timer circuit and where I can get a suitable telecoms changeover relay or Y adapter for the video lines?
read that several times and think it sounds like a plan. If I build a timer with a double pole relay you can use one pole for a 12v feed out to switch the camera feed on, and the other set of contacts to put the video feeds through. So it will be an all in one timer and changeover box. I'll sketch a plan out later 
.! No, I shouldn’t think there will be a prob as I recently split the video wire on a reversing camera mounted on the sep bike rack and connected the 2 halves with a 7 pin trailer socket. Image is fine. But shall we see if Lenny HB has any thoughts first? Otherwise good to go.
