Kubota tries to build one wiring harness for multiple options. My tractor has a couple empty, hot when key on, plugs that I've found. I used one to power a work light.
If the circuit you found becomes hot by simply turning on the key it's not going to work very well for a back up light. You would have to cut into that circuit at it's source and power it thru a switch on the dash. Before I did that I'd simply create my own circuit. We also don't know how many amps of draw that circuit will handle.
Do you have a service manual for your machine that has a wiring harness diagram? If not, maybe go to your dealer and ask them to print you a wiring harness diagram. It will come in handy in the future.