Tach fails lights turned on.

I have a '79 vette and am having a problem where the tachometer pegs all the way to the right when the lights are turned on. If I turn the dimmer switch all the way down so that the interior lights shut off, the tach will work. The headlight switch was just replaced but that didn't fix the problem.

Has anyone heard of this problem? Better yet, any ideas on the cause & solution?

Thanks