Typically the two lighting bulbs in the instrument cluster work from the same power supply. if only one went out I would think it is only either a problem with the bulb itself, the bulb socket, or the board where the specific bulb is mounted. I can get you the wire diagram if you still need it and tell you the pin that runs the lights, but it is more complicated than that since the circuit that does the instrument lighting is continuous with the dimmer rheostat and the rest of the interior lights making it more involved than just applying power to the correct pin. The circuit is incomplete with the cluster out of the car. You can apply power directly to the holes in the back side of the actual bulb holders on the back of the cluster to make them light, typically both lights will turn on when one is powered. Just insert probes into the back side holes on the bulb holder and the bulbs will light when power is applied to test the bulbs

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Christopher Henkel
1990 190E 2.6 - Arctic white SOLD
1986 190E-16v - Blauswartze
1993 300CE - SOLD
2003 W208 CLK 320 Cabriolet - Magma Red