1/3 of Display is dim... Which part to check/replace?

I have a 2007 Aluminum iMac with a display that dimmed in the upper 1/3 of the screen (suddenly, a year or so ago). I would guess it's about 20% dimmer and gradually gets to full brightness in the bottom 2/3.

I was just about to buy an eBay LCD and follow these awesome iFixit instructions when I saw that there might be other causes to this...

So my question is: What is the easiest way to diagnose this issue? I've read here that it could be the inverter, the CCFL in the LCD, or even a loose wire. Has anyone else had this issue and gone through with a repair? If so, what ended up being the cause?

If it was the LCD, could I replace it with one from a 2009 iMac?

I was also planning to upgrade the HD so I'm looking forward to taking this thing apart... but hopefully as few times as possible!

Thank you in advance for your ideas!