Volvo Repair: Dash lights, signal work, dash lights

Question
I have a 1990 GLE sedan,16v engine, nonturbo, it has 182K miles on it. I recently noticed that my instrument lights and gear shifter lights are on all the time, except when I turn my  headlights on, then they go off? My brake lights, headlights and my turn signal work and I also checked the fuses. Is there any thing else I forgot to check or should check?
 Thank you for answering the forum questions. If I had any spare money to give...I'd give it to you. Would you take a pat on the back and "outstanding job" instead?

Answer
Bruce, Pats on the back are always appreciated.  Thank you.  Thats a strange one, it should actually be opposite.  I've seen this before but of course now I cant remember what it was.  If you have a spare headlight switch, I would try that.  The other possiblility is the dimmer switch.  Let me know how it works out.  Roger