Curious if you were able to see any signs of damage on the old camera when you took it out? Anything at all to suggest what the cause of the original problem was?
Obviously the camera being on the back of the vehicle it will attract dirt and need to be cleaned, but if there really was an issue with the focus that would be very interesting. If the chip was damaged by sun exposure, does it not have a shutter to protect against that damage?
Cameras without user-serviceable manual controls to adjust these things certainly create a significant point of failure!