Dell 1520: Caps Lock, Num Lock & Scroll Lock LEDs not working?

I have a Dell 1520, out of warranty. I broke the hinge supports and had to replace the lid, meaning I took the LCD out of the old lid and installed it in a new lid. I had no problems, except that the LEDs for the Caps Lock, Num Lock and Scroll Lock don't light up. I wasn't really even messing with that part of the laptop, except to take off the cover to get to some wires.

What could make me lose these lights? The keys work - just not the lights.