Hmmm! So my cluster has two 'knobs' - the one of the left resets my trip odometer if I push it in (but you're saying this knob also twists for the rheostat?) and the knob on the right, to set the clock.

I'll be out for a drive later today so I'll give the thing a twist and see what happens! Hopefully it's that easy.

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1984 300D Turbo, 326000 Km, Thistle Green Metallic