JC, you are absolutely correct. The power source for the light is from the license plate light. When I installed the GTGuys hatch opener I had to move things around a bit, but it worked fine. AnalogDesigner's light is a high-output LED.
Would you have any photos of the fixture and how it mounts with your GTGuys latch opener? If this is no longer a viable option, perhaps something new could be developed with a bit of research.