The way mine are set up on the '87, is the knob to the left of the steering wheel does the instrumentation brightness by rotating, and if you push it in, will reset the trip odometer. The right knob does the clock (you have to push it in for it to change the time).

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1982 240D, sold 9/17/2008
1987 300D TurboW124.133 - 603.960, 722.317 - Smoke Silver Metallic / Medium Red (702/177), acquired 8/15/2009
262,715 and counting