Wiring in a second camera ,

My van already has a camera wired to,one of those 7 inch clip on monitors , says it has dual port , V1 and V2 , how does the wiring work for the second camera , as there does nt seem to be additional wires or port to hook in second cam on the monitor

Follow the cable from the monitor back into the dash. Between and a half to one metre, you could find a junction of wires, positive/negative wires, input video feeds, etc. It is a this point that you connect the second feed for a camera.
Usually after remove the panel under the steering this junction of wire is placed on top of the wheel arch or cable tied to steering column.
Just be careful, camera feeds come with about 3 different connection plugs. When you find your connection, post a photo here and someone will identify it for you.
There is another option, buy a wireless camera and bluetooth it your phone and the phone becomes the monitor.
Malcolm