I'm guessing the grip assist is the missing handle up around the top of the driver's window, something to grab hold of to swing into the cab, nothing to do with the footrest I'll bet, the Forerunner I had had one standard, you can see where the Navara one should go.
I've pulled back the carpet next to the clutch and found 2 holes about 100mm apart. one is higher up and the other is down around floor level. Both have black plastic plugs blocking them. i am guessing that these are the mounting holes for the foot rest??
Does the foot rest come with the nuts and bolts that fit?