First make sure you don't have an intermittent contact on the fuse for
the instrument lights.

If that's ok, It might be related to the dashboard brightness
adjustment knob.
This knob is moving a potentiometer ("Pot") - this is basically a
variable resistor, much like the volume adjusment on your stereo. This
pot is usually hooked up to a regulator that sets the voltage to your
dash light bulbs, dimming them.
That functionality is usually contained within one module. The
archilles heel of this whole adjustment is the pot.
It probably has a dirty runner (the part that moves when you turn the
knob).
Try moving the setting vigorously back and forth to see if it cleans
itself. It might make it worse, in which case you will have to replace
that particular module.
If you have soldering skills or know someone that does, you could see
if the module opens up and check for bad solder joints and replace the
pot if you can find a direct replacement.
You could, of course, also just replace the module.

If you don't care about adjustment and just want to set the brightness
to full, check the three wires going to the module.
Identify which contact goes to ground, which to 12V (easily identified
with a multimeter). The third wire is the one that goes to your
dashlight bulbs. Tie this third wire directly to the 12V wire.

Remco