The cluster light on most cars go through the headlamp switch. My 68 has a separate dash light/dome dimmer switch. The dimmer on the floor can be a problem. Connector and switch, usually get wet and full of gunk. Cheap enough and for whatever reason I've replaced a few turn signal switches and harnesses. Ya run a dedicated ground to cluster. Usually a few screws are grounded on the PC board. Pick 1 that's convenient and run a ground. Good luck.
