No more instrument panel dimmer?

The other night while out driving my '70 Duster the instrument panel lights all of a sudden went out. I thought it was a bad fuse but a new one didn't fix it. I then found that the panel lights would only work if I turned the headlight switch all the way to the left. What happened, and how can I fix it? I put in a new headlight switch not too long ago so I hope I don't have to get another one.