The V-sync is the first one to try and yes, it is a PITA! Some have just left it disconnected but I managed to flatten the contacts back out and separated them. Still has a little grumble mid-startup but no more grey screen. Just be sure the LCD cable is installed correctly too.

Just to resurrect this old thread, I’ve just updated to an SSD in my late2009 21 1/2” imac, (yes, it was still running the original HDD!). I have fat fingers and it took me ages to get it to run again. Anyway, I had exactly the same issue with the vertical sync cable. Bent wires which I managed to straighten again and hey presto! I can actually dismantle the thing with my eyes shut now. :-)