I wish I could tell you, it sounds like it should work. Just give it a try.I had thoughts of adding additional reverse lighting in the future, so when installed my trailer hitch wiring, I soldered on a lead to the pigtail/extension on the trailer wiring harness's reverse light wire. The trailer hitch wiring has a converter box which already has 12V running to it, so adding a relay to power additional reverse lighting would be easy peasy.
After reading the posts here...I'm a little confused whether the CAN-BUS is going to let me do this. The only additional load on the stock reverse lead would be that of the coil in the relay. Is that enough to upset the C.B.?
I’m glad I had to wired with a switch. That way I can control when I want the lights on.