How to repair iPhone 6 plus 8MP iSight Rear Camera?
Replace my iPhone 6 Plus 5.5" Inch.
This is a belated answer so hopefully it can be of use to folks who search for and find this question.
Unfortunately, the link no longer works but generally speaking, any camera related issues are usually solved by replacing the camera module. These are easy repairs when you follow the right guide.
In certain circumstances, the issue is at the logic board level and requires a micro-soldering repair but the first step in troubleshooting is to rule out modular component issues first.