Display Backlight won't dim after HDD replaced. Brightness near 100%

I put an SSD in my Early 2009 iMac 24" after the old HDD failed. I don't recall damaging anything during the repair, but now I've got a problem with my display.

The backlight doesn't want to dim. Lowering the brightness all the way only dims the display slightly, so 0% looks more like 95%.

Resetting the SMC and the NVRAM didn't help, and I can't find any solutions online.

My parents are the primary users of this computer, and their elderly eyes are having a pretty hard time. It's very straining on the eyes, especially with black text on a white screen.

I'm hoping that I can figure this out so I can make this computer easier for my parents to use. I know it's an older machine, but it's still a great computer. Plus the SSD has given it a new lease on life.

Any help would be greatly appreciated!

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