Yramb,
I am very interested to know more about your installation. I have been trying to get a winch fitted in my 2012 Prius for some time, weith no success. A few days ago I met up with an installer who was very approachable and seemed to know his stuff. He checked the boot of the Prius carefully and concluded a winch could not be fitted. One problem was the slope of the back of the car, but this could be overcome by using a folding winch. More serious was the floor of the boot area. The floor is a lightweight board which covers a storage area, below which is a compartment containg the spare wheel and below this is the battery area. The guy reckoned this made it difficult to get to a rigid area to mount the base of the winch.
Can you please give me details of your installation. eg what was the make and model of the winch and contact details of the installer? Also, are you able to see how the winch base is fixed to the car? I am really very keen to pursue this.
(If I could find a private messaging facility on this forum I would send you my email address to make our exchange easier.)