Rear-Facing Dash Cam: Where To Mount In/on Pickup Truck?

Not a backup cam. A dashcam with high rez that records what it sees.
Seems like the rear window of a pickup cab does not work. The bed gets
in the way and the cam is not high enough - so that somebody rear-ending the vehicle would not be sufficiently imaged.
Or is it?
Has anybody been here with an actual installation?
I would like to hear that the rear window actually does work.
Barring that, I'm thinking maybe cutting a hole in my rooftop box and mounting it in there. viz:
Pete Cresswell

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