I assume the Op wants to turn on his turn signs and still see the rear view in his monitor not have it automatically switch to the side view camera.
Personally I wouldn't do this as there is nothing to see in the std rear view other than the front of the toad. That said wouldn't disconnecting the turn signal wires to the system accomplish that? don't disconnect the Cameras themselves . That stops the automatic transfer of the monitor to the side cameras and still leaves them avail via the monitor controls. The system I have can't be disabled if the turn signals are wired into the loop.