Dash & dome light question

I have been tweaking the last bugs on my 67 Barracuda. I am left with non working dash and dome lights. I have checked power to all my glass fuses and they all are good and power both ends. The relevant fuse is the Inst. fuse which with ignition on and headlight switch dimmer full left gives full voltage to both ends of the fuse and when turned to the right the voltage drops per dimmer protocol. I am thinking grounding but I am unsure where to ground to. I bought a new headlight switch which has a tab that the original one does not have. Does the headlight switch have to ground for this to work. All the power is going where it is supposed to. I am stymied-------------