Dashboard Lights that won't dim

DASHBOARD LIGHTS: When I initially start my 2006 Prius and turn on my headlights, I can control the dashboard dimmer lights -- for about 10 seconds. After that, I have to turn the dimmer control all the way to the "lock" position (brightest). If I nudge it out of lock, everything is dark. Ideas? I've checked fuses inside and out, and all seem OK.

PS - If it makes any difference, I have no lights at all coming through on the AM/FM, Seek/Scan, etc. buttons on the radio.
Any helpful hints would be appreciated!