I used a Single Pole Double Throw (SPDT) switch to produce a similar set up on my bed lights.
Center terminal goes to the lights, one side comes from a constant 12V source, the other side comes from the overhead lights. I can select "on" at any time using the constant feed, "switched" which will turn them on and off with the cargo lights, and "off" which which is self explanatory.
Using a relay to pull power from the battery is a good idea, but I was only powering some LED strips so I didn't want to mess with the additional wiring.