If you have a compatible Garmin you can use a wired or wireless backup camera. The babycam isn't waterproof but is completely wireless so you could mount it in the rear window or outside if you don't drive in the rain. Garmin also has wireless backup cameras that mount to the license plate frame and need only 12V.
Babycam - says discontinued but maybe you can still get them on Amazon.
https://buy.garmin.com/en-US/US/p/521255
This one needs 12V only so the video data is wireless. And its waterproof. Looks like its compatible with lots of devices you may already have one.
https://buy.garmin.com/en-US/US/p/501486