I'm interested in the same thing and have pondered way to do so.
Potential Plan A:
1) Take the bolts from the rear seats and find some big washers(maybe overkill) that will work with them.
2) Find a grommet that will accommodate the bolt diameter and press it into the distal end of the harness (homemade or bought-undecided).
3) Put the bolt with washer through the grommet and into a tub hole where the seat was.
Potential Plan B:
1) Loop the harness over top of rear Roll Bar (I have a 4door)
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