Had this on the 66. My dashlights have push in sockets with metal fingers that ground to the metal on the back of the cluster.
The cluster had a metal strip that fit under one of the cluster screws and one of the cluster to dash screws, grounding to the dash.
The dash must have a good ground to the body, and such to the battery.
When I got done, I had dash lights with both the old headlight switch and the new one.
I elected to install the new one.