back up camera won't work in reverse

How many cameras ?
What camera is on while driving forward ..side view...rear view
Maybe the rear view camera is dead

You can also wire them to stay on when your driving forward too!

.

You have rear view while driving forward, but it goes blank when you shift to reverse
That msg means ...no camera input attached to the secondary input
The signal on the reverse switch
Shift means it wired to the backup lights ..as sensor mode but no camera is attached
You have two choices moved the camera aka video signal to the secondary input and have video in reverse but not while driving
Or disconnect the Jensen from the backup signal
Find the manual and disconnect that wire and the monitor should not switch signals
There is an easy test
Find the fuse for the backup lights and remove it
Then switch to reverse and see if the display stays on
Obviously you want to keep your back up lights but do this as a, test in the driveway

If you do not have a manual, here is a link to one:
Read about Camera Triggers and Priority:

Oops, the forum link tool isn't working for me.
You'll need to copy/paste.

http://www.jensenrvdirect.com/downloads/dl/file/id/574/jrv212t_owners_manual.pdf

Obviously you have to switch the pos and neg wires on its terminals! :)