Help fitting a dog guard

I've just bought a second hand dog guard (no instructions) and wondered if anyone could give any hints about fitting it...
Oh, it's for an Estate (Wagon) if that makes a difference.
It obviously clamps to the mounting points in the load area but it also has mounts that look like they're supposed to screw into the rear door pillars. Taking the trim off the door pillars, there's no screw thread for them to fit into. My only idea is to drill through the trim and then tap a thread into the pillar. I'm not sure the steel will be thick enough to hold a thread though.
Any ideas about the pillar fixing or does anyone know how it is supposed to eb done?
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