For anyone still here and looking for an answer, I posted here years ago about resoldering the connectors on the backlight strips as a first step to repiaring this issue, and complained about no one believing me that this was the fix.
Last week one of the machines that I posted this fix about came back to me, a 2011 27 inch Imac.
Its still going strong on its original EVERYTHING. It came back for a GPU upgrade. I personally have a thunderbolt display, and a 2011 27" Imac that I use for reading schematics in my workshop, both given to me due to this issue that have been running on all original hardware for 4 years now.
These and about 20 other machines I repaired for customers of the 2009 to 2013 era all had the same issue, and were repaired the same way and are either still going or retired for newer machines.
Save your money and resolder the backlights before ordering parts. if that does not fix it then get the backlight power supply.
I deal with odd out of box fixes all the time, and for some reason just like the mid 2012 macbook pro ram socket issue I seem to have alot of trouble convincing people to do the cheap fix first.
Have fun,