It doesn't matter what camera you have, the image darkness is inherent to MCU2.Can you give more detail about the rear-camera? I have the 3rd gen rear camera (looks fish-eye-ee) with MCUv1 and I found that it was a slightly better picture with better night-vision. But are you saying MCUv2 screws up the image or doesn't look right with the older camera?
And I've said this a bunch of times, but once again, it's only darker at night. During the daytime its functionally identical. It must have something to do with the post-processing on the image done inside the MCU2 hardware.