You most likely have a loose electrical plug on the back of the dash. Fashion a stiff wire with a small hook in the end, and use the hook to wiggle the dash cluster out enough to disconnect the wires on the back of it (to entirely remove the dash). The large electrical plug is most likely loose.

Regarding no dash lights, naturally check the two dash bulbs first. They are at the top of the dash cluster. If they are fine, your dimmer is probably bad. The dimmer has only two soldered connections to the circuit board...it is pretty obvious which two they are. If you solder a short insulated wire across the two connections, you will have dash lights at the brightest level, which is about a normal setting for me

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'84 300D ~90k
'82 300SD ~171k
'82 240D ~175k
'96 Ram 2500 with 12V Cummins & 5 speed ~130k