LEDs dim when the voltage drops, and the simplest LED wiring is with a resistor and LED in line. yours probably has a more sophisticated circuitry, so, it could be one of the components going bad... components that can usually go bad are electrolythic capacitors, as the juice inside of them may dry up and thus changing the nominal value...
Is the circuit board accessible? 2.5 years later, it is probably not under any warranty - how much is it to get it fixed by the manufacturer?