Try armorall. I had a 83 300D with a beautiful dash that glisened. I kept it that way by rubbing it down in armorall at least once a month. The only reason a dash will crack is if it dries out.

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KCScout
98 S420 My Wife's Baby
97 S320 My road car
83 300D My old road car (love that diesel)
81 300SD Wife's former Baby
81 240D A fun stick shifter
71 220D My first one