Please help- My Dashboard Won't Light Up At Night

My 1998 Old Silhouette has one of those twilight sensors that turns on the dashboard lights (Speedometer, gauges etc) when it gets dark. Lately, at
night, my dash won't light up. I've check the fuses, played with the headlight switches, banged on the dash (not too hard of course), but nothing seems to work. Sometimes, when the engine is off and I remove the key from ignition, the chimers keeps chiming to tell me I left the key in.
Does any one know why this is happening?
Thanks in advance.
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