Can do something even simpler for cars with rear window wipers.
Take bike brake line, attach one end to the rear window wiper, and route the other end down to where the camera is and use that motion to run smaller wiper over the camera lens. Probably set you back <$10.
For cars without it, use a 555 timer in one-shot configuration+mosfet/relay to drive a small 12v DC motor that is all powered off the reversing lights, so it automatically does 1 wipe every time you shift to reverse. If you didn't want the wiper to spin 360, you can adjust the pulse width of the 555 and use a spring to return. This would also cost about $10.
Or just get a plastic bottle and craft a small cover to stop the rain getting on it. Mine is angled slightly downwards and is covered (the license plate part and camera are recessed into the body a few cm), so when it rains it's not an issue.
Take bike brake line, attach one end to the rear window wiper, and route the other end down to where the camera is and use that motion to run smaller wiper over the camera lens. Probably set you back <$10.
For cars without it, use a 555 timer in one-shot configuration+mosfet/relay to drive a small 12v DC motor that is all powered off the reversing lights, so it automatically does 1 wipe every time you shift to reverse. If you didn't want the wiper to spin 360, you can adjust the pulse width of the 555 and use a spring to return. This would also cost about $10.