‘66 Cuda Won’t Turn Over - Only Got 6V at Switch

I don't know where/what the problem is. I'll type out some info that may help determine where the problem isn't...
You stated you were jumping the starter relay when you created a short circuit. So the ignition switch had to be in off position at that time to prevent starting. All switch contacts open stops any fault in the wiring right there. So switch itself is not the problem.
Why are you reporting reduced voltage on some wires in some switch positions?... Current can/does flow through the ballast resistor in both directions. There are 2 wires involved, a blue and a brown. Any time the ignition switch contacts pass 12 volts onto one of these wires you will find reduced voltage on the other of these 2 wires all the way back to the ignition switch.
there are other causes of low voltage... if your fusible link, any wire, or connection is fried to nearly open, you will see 12 volts, just not enough of it.
So get 12 volts into the ignition switch and out of it on the yellow wire, enough of it to energize the starter relay.