Rear facing camera failure

Good day boys and girls. I have had an ipad 4 come in with a non-working rear facing camera. I have tried quite a few things, which I will list, with no success or change. Please note that the front facing camera works perfectly, as does the headphone jack. I am looking for some tips as to where to look next for the root of this fault.

Here are the things I have tried:

1. Hard reset (power + home): no change

2. Reset to factory defaults using DFU mode: no change

3. Replace rear facing camera woth known good unit: no change

4. Replace upper io board with headphone jack, FF camera and microphone: still no change

I have inspected the zif connectors for this upper io board under my microscope and they look pristine. No evidence of drop damage or water damage. Also no evidence of any previous repair attempts.

Any help with this would be absolutely smashing.

Thanks guys