There really are only 3 of them. You can check out any of the threads on replacing the stock gauge lights with LEDs and find pictures showing which of the bulbs are the backlights.

Just replaced the bulbs. I perhapse blamed the previous owner a bit too quickly on this, the green covers weren't removed they were completely disintegrated by the heat of the bulb and they no longer did their job. There are 3 back light bulbs too.

Behind the dash I found two loose sockets, one I think is the remnants for the diff locker wiring that won't be populated if your 4runner didn't get that option. The other socket I can only guess might of been for ABS had my 4runner gotten it equipped.

When removing the dimmer knob twist it completely to one side, note its orientation, and then remove it. I almost went back into the dash because the backlights weren't working, I forgot that you can completely turn them off using the dimmer. The only tools you need are 10 and 12 mm sockets plus a Philips head screwdriver, really simple.

Turns out it does have a separate part number, but the price is the same or higher than the complete piece, so no point really.

Part # 84998-10720 1999-04 Bulb Cap only