Take the cluster out, check the back of the cluster for a burned fuse, which looks like a red lighbulb, if it is just replace it. While you are there, you can also check the contacts of the dimmer with a multimeter. You can replace it yourself (two screws and a $30 part). You may/may not have the fuse, based on when in 86 you car was built. Starting on 87, according to the manuals, they do have it. I did not have it in mine. I ended up taking my cluster to Steve in Gainesville, Fl where he found a couple of blown wires in the board. All fine now. BTW, mine happened after I changed one lightbulb. Did you do anywork on your car?
If you did not do any work in your car, most likely is your dimmer...

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J.H.
'86 300E