Hi,
I have the same (I assume there is only one) type on my commercial that was there when I bought it.
Having had the rear door panel off to run some wiring I can remember thinking that the guards shouldnt be too hard to fit, although I may be wrong.
Take the rear door trim off, 3 or 4 screws along the bottom edge and then pull from the bottom and sides and may need lifting clear at the top.
Once in there im hoping that you would see 2 pillars that are square that are runninng vertical at the top of the door (right under where the top of the trim would be). These 2 pillars would have 2 threaded holes in them.
Now, if thats the case then all you need to do is make 2 holes in the top of the door trim same centers as the square pillars and 2 on the facing that are the same centers as the threaded holes. The later been for thumb screws to hold the guard in place.
I am purely guessing at this and hope that someone can correct me but its worth taking off the door trim just to have a nose, at the end of the day you only need a phillips driver to look and maybe a drill to make a couple of holes.
Steve