Replaced the dash in my 1984 300SD this last summer. Lots of slip fit, hidden and generally German fittings. Don't force anything! (odds are it's a hidden connection you overlooked). Remove anything that might remotely interfere. A roll of white adhesive labels and a black pen to label everything for reconnection. (it all looks different when you're putting it back together) If your climate control is working fine, I would replace the three? hidden vacuum actuators; if climate control is not working, I would really try to diagnose as much as possible first, so you know what to double check when you are in there. Allot plenty of time... it aint quick.

Use 303 Protecterant on the new dash to help preserve it.

PartsShop/Fastlane has the vacuum actuators.

Definitely take pictures (for your ease of reassembly and for us!)

Replace all bulbs while you're there.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)