Part of the stove control backlight went out - logbook Toyota Camry 1994

The other day, part of the backlight of the stove control went out, I redid the backlight myself with the help of super bright neon glow diodes 3 volts, 4 pieces are connected in series in a circuit to get 12 volts, they are included in the base of the standard backlight!
Since the other three lines of 4 diodes are burning perfectly, I think one of the diodes has died!
Either a low-quality diode was caught or some kind of power supply problems, it seems like diode bulbs are almost eternal, but here they burned out for only 2 years!
Expensive for a radio electronics store ...