12V switched power source in 1968 mustang

I tell you what I did for a switched power source. Instead of tapping directly off the back off the ignition switch I ran a wire to a relay from the back of the ignition switch and on the otherside of the switch I ran a wire to ground. On the other side of the relay I ran a wire to a constant 12V source (starter solenoid in my case) and then on the other side of that switch ran a wire to the accessory that need the switched 12V source. Although in my case I ran it to an aftermarket fuse box so I could have multiple switched power circuits I could tap off.

So then when the key is in the "ON" position, you complete the circuit between the wire from the ignition switch to ground which closes the switch connection your 12V source to your accessory. Since you're not drawing power directly off the ignition switch you'll avoid premature wear of the ignition switch. These cars weren't designed to have so many sources pulled off the ignition switch after all.

I added a 40 amp relay also. I set up fuse blocks that were switched and unswitched.
Here is a schematic from my webpage:
Aux 12V Power

Couldn't you just take a wire from the "I" post of the starter solenoid? Ues an inline fuse and it should be good.

My 68 had a little unused plug under the center of the dash that had a 3 female plug. It is a switched 12 volt. I'm not sure if it's factory or not, but it looks to be.

The "I" post on the starter solenoid connects to the pink resistor wire and it also goes to the coil. The resistor wires reduces the voltage and is not really 12 volts. MSD ignition boxes have problems if you connect to the I post because of the voltage drop needed by the original coil.
Im unsure if Duraspark has the same issue or not...

Why wouldn't you just use a terminal off the fuse box that's on a switched circuit, like say, the radio?