1990 420SEL Warning Lamps

There is an unmarked warning lamp in the Warning Lamp cluster. The owners manual does not define it, although it shows on the instrument
panel illustration. The lamp in question is on the right side between the SRS lamp and the Check Engine lamp. The car has been sitting for a while, and when I started it this AM this lamp stays lit. Don't dare drive it until I know what it's telling me!
Does anyone know it's function??
Thanks in advance.
John S.
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