Consider retrofitting a beeper for when you back up. Your vision is limited to begin with. Besides, these days, with the way the roads and driveways and job sites are crowded, it's a nice-to-have. (They do save lives.)
Here, in Philly, it's not uncommon for some of the vans that do a lot of work in the city to have a rear-roof-mounted mirror. 'They say' that it's mainly for seeing if anyone is back there when they stop at a traffic light OR before they get out of their van. Contractors have been known to walk around to the back of the van, after just getting out of the van, and finding a gun pointed right at them! (It wouldn't serve much purpose for much else, but maybe.) I just thought I'd mention it.