WHen you turn the park or headlights ON, can you vary the dashlights/radiolight intensity with the intensity control up to max?
www.avigex.ca/xport/interiorlightsdimming.jpg
radio light (radio ON and key to INST or run) supposed to be to max when headlights are off, then dim to set intensity when headlights are turned ON
The dashlight dimmer is a variable resistor and this component can erode or fail with time at intermediate settings but chances are it will still work when set at MAX....
ALso there is a control transistor in this circuit....but generally it is sturdier than the dimmer button itself but it can fail too.
Finally al sorts of ground failures can explain this....
...make some tests and report.