Reach up behind the dash, push it out, and remove the ceramic part from the left side behind the knob your freind was messing with. Now you have a choice to make. You can (1) replace it with a new Dash Light Rheostat, or (2) turn around and throw it as far as you can, and then jump the two metal posts remaining in the dash there with something that will adequately conduct 12 volts (many solutions to this) and push the dash back in place.
That is a common failure on these old diesels. Easy enough to take care of. I used push on pins from Radio Shack and a short piece of stranded wire to get the dash to light up again. You will lose the ability to dim the dash lights without that part, but that is up to you.
BTW, the part is available here at Fastlane for $30.79 #P3012-11761. Locate it under body electical to get a look at it.