Up for sale is a good functional Volvo VNL dashboard light dimmer switch. 

This is the unit that dims the gauge cluster lights, located to the left of the gauge cluster.

Take a look at the picture to be sure if this is right for your truck.  Good used unit from a recent crash damaged truck - comes with a 30 day warranty.  I have tested this unit for correct resistance with an ohm meter and it works properly through it's working range.  Do NONE of your dash lights not work, or do they flicker when you try to adjust the brightness, or only work on full bright, or only occasionally when you fiddle with the dimmer switch?  All of these are signs of a possibly defective dimmer switch. It will come to you as seen. These are starting to be hard to find in good working condition, $122 list from Volvo, 1/5 that here. Bargain alert!!! Keep your classic Volvo going.