I have used rental car minivans to move before, years ago natch, when I had less to move.

One episode I remember involved a Hertz minivan, where the rear seats were locked to their anchors with some type of keyed padlock. A quick trip to a locksmith provided the solution, and I left the rear seats in Chicago while I moved my belongings.

Is there some language in the contract indicating that the customer should not remove the seats?

BTW, minivans have by far the most storage room (compared to sedans or to SUVs) in terms of rental cars.