You will have to install a minimum of 4 to get decent coverage of the back................I know you don't want to install a camera but I think that this will be more effective. I intend to install a camera which will be liked to a monitor in the same location as the original mirror and be on all the time giving a lot better view than looking through the engine louvres.
I installed a reversing camera on the Vito Van and far better than any reversing sensors in my opinion..........
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