I was looking at one at Wally World that fits onto the license plate holder and has it's own 3 1/2" screen. They're under $100 and look like they'd do the job. You just have to find 12V power and run the cable. I've thought seriously about getting one to replace the rear-view mirror that blocks my forward vision somewhat. . .
Of course, for just backup use, you'd hook the power to the backup lamp power.