Yeah, there's really no way to do this without removing those trims. I removed the trim from the tailgate as shown on page 2 of the instructions. Also 2 other trims, the first one on page 3 and the last one on page 7. I was able to tuck the wires under the weather seal of the D pillar and then from there under the trims all the way to the front to the footwell.
One thing I'd recommend though is not to put back the tailgate and the middle headliner trims until you're absolutely done so you don't have to remove it again to troubleshoot if needed. If you're the only one who drives the GC then no one would notice that the back is bare
Some pics of after the install.
Camera mounted without the cover.
Camera with the cover on.