Slow resurrection, but still can't get diagnostic LED #4 to light up.

Buckle up, this is a ride.

I was given an old Mid 2007 iMac 20". The previous owner had used it daily until about six months ago when, according to her, it just died. It was working the night before and the following morning it wouldn't turn on. She bought a new machine and asked me to recover her data from this one, which I did. She said I could do whatever I wanted with the old machine. Since I had a number of spare parts available I decided to see if I could resurrect it in my spare time. This has now become a mild obsession. :-/

At first I got diagnostic LED #1 and that was it. No other signs of life even with everything disconnected. So I replaced the logic board with a known-good board.

Then LEDs #1 and #2 lit up and gave me a startup chime, but no LED #3 or any other activity. Bad GPU, seen it before. Okay, no problem, I have a known-good video card I could put in.

That gave me LEDs #1, #2, and #3 as well as the startup chime, but no LED #4 and no backlight or image on the LCD. Bad LCD panel? Strange that all of this would die at the same time but okay. I borrowed a known-good LCD panel from a friend and swapped it in, but LED #4 resolutely would not turn on and I continued to get no backlight or image on the LCD.

I asked my friend to test my "bad" LCD panel on his system and he says it works perfectly, if not a little on the dim side (common for this machine).

I tested the internal video cables from both monitors with a multimeter (which took an insane amount of time) and they're both fine.

I get no video through the mini DVI port, with or without the internal display connected. The monitor I have it attached to stays asleep so there's no signal going to the external display at all.

The power supply's output is within spec for the machine. I also replaced it with a known-good supply and that didn't solve anything.

The CMOS battery is brand new.

I am able to zap the PRAM on startup but that doesn't solve anything.

I know this is just a 17 year old iMac and not worth anything and if I had any common sense I'd just scrap it altogether. But I hate unsolved riddles and this one's really bugging me. It feels like I'm overlooking something simple but for the life of me I can't figure out what it is.