My instrument cluster (85 vanagon with auto tranny, with tach)
recently began exhibiting the following symptoms:

Engine running.
Headlight switch off:     All is well.

Headlight switch on (either Parking lights or Headlights):
   Parking and/or Headlights turn on as usual.
   Water temp gauge drops to zero.
   Tach drops to zero.
   Clock turns off.
   High beam LED glows dimly. (if i turn on high beams, the LED goes
OUT, ironically)
   Cluster illumination lights off.

If i rotate the cluster illumination dimmer control, the tach & water
gauge will flicker to life briefly.

Headlight switch back to OFF:
   Everything goes back to normal.
   The water temp gauge blinks for a few seconds (as if you'd just
turned on the ignition).

Is this more likely a headlight switch failure?   Or perhaps ground
problems in the fuse box?  Or the voltage regulator issue?

thx!