SOLVED: iMac powers on with LCD disconnected

Have a 27” Mid 2011 iMac in for repair. 

Customer asked to have it pimped out with an upgraded graphics card, RAM, SSDs etc.

Did the work and put it together. Powered it up and was working fine. Issue is that it will give a short fan spin with one LED lit and not start unless the power has been unplugged for a number of hours. I reduced the iMac down to minimal config (everything non essential disconnected) and found that it will power on fine with the LCD disconnected.

I swapped the LCD assembly and PSU with known good ones and this made no difference being the most likely parts.  

I have also swapped;

Logic Board,

Graphics Card (Brand New like the one I one I originally installed)

RAM

Backlight Board

Wifi card

Each of the swapped parts are known to be working fine and have been tested in another iMac. The graphics cards are brand new from Dell and have been flashed by myself for iMac compatibility. (I have installed 25 of these cards without issue)

Whats the next logical step forward on this repair?