In my 83 300SD...
on the far left is a knob - it resets the trip odometer by pushing and dims/brightens the gauge lights by rotating. This knob connects to a rheostat on the back of the instrument cluster - rheostat may be corroded or the spring giving variable resistance may be broken. Mine had (2) 3 watt bulbs to illuminate the gauges - they may be burnt. There may be corrosion on the brass strips under the cream colored 4"x6" box that holds the gauge lights. Also, where the bulbs seat - look for corrosion there.