Just as an aside, I was able to connect it to the running light wire on the trailer plug so that it's still switched, but it doesn't have the "boot up" time delay from when it was on the reverse light. It does mean that during the day if I don't have my lights on it won't turn on, but until I run a dedicated switched lead from the front it's good enough for now.
Thanks for the help earlier, and if anyone else knows of a ignition-linked switched point near the bumper that would be great.