dash light problem

Hello all, Please dont laugh. I have a 1975 Ford Courier mini truck. A problem has developed. When the key is turned on the dash lights go on, when turning on the headlights, the dash lights go off.Any ideas what the problem wouold be? Thanks  jimka2@hotmail.com

I'M NO EXPERT BUT SOUNDS LIKE A BAD GROUND SOMEWHERE IF U CAN'T FIND IT TRY RUNNING AN EXTRY GROUND WIRE FROM NEG.BATTERY TERMINAL TO THE BODY LET ME KNOW IF I WAS OF ANY HELP

Thanks David for the info. I tryed installing a new ground wire from the battery to the frame, and also the engine block. No luck. However I have discovered that the black and small white wire that has power to the radio, also sends power to the dash lights which is my problem. Any new direction for me to take ?? Thanks Jimka

take the fuse panel loose and make sure the wire coming from fuse panel has current my rrother had that problem on his tail lights the fuse was good but he had bad connection on the back side of his fuse panel hope that helps