is the monitor that goes on off or the camera
the camera gets its power from the monitor, if the cable is good an the camera is good, then the problem is in the monitor
if the monitor goes on off, and the testers pronounced it good
then the ground is in the wiring going into the monitor
the manual papeer or online should list which wire is battery negative
find it at the monitor cable
cut it
run a new wire from a know good ground
splice together
it is unlikely that the monitor ground is "all by its lonesome"
if nothing else is giving your trouble then, its a connection in the harness
a new wire is the best fix
I can explain it to you.
But I Can Not understand it for you !
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1997 F53 Bounder 36s