12volt switched power?

The + coil wire and the I terminal on the solenoid are the same. They get their voltage from the resistor wire off the Ignition Switch C (coil) terminal.

When cranking though, the solenoid provides the full voltage to the coil through the I terminal. Once running the coil receives 6-9 volts because of the voltage drop across the resistor wire.

You may measure 12v at the coil or I terminal with the key on and the engine not running. It depends if the points are open or closed. If the wire is removed from the coil, same thing, there is no load on the resistor wire, so there is no voltage drop. Get it?

For the MSD the main current draw is through the heavy red wire. The smaller red wire goes through the switched source (coil +) and does not draw alot of current, so leaving the resistor wire in is fine. It's just a "turn on" signal.

If you are wiring an electric choke (or other high current device) however, you would have to go through the firewall directly to the "C" terminal on the ignition switch. I would use a relay to prevent excessive current draw from that terminal though.

Learn something every day. I would swear I saw 12v with the car running, but it wouldn't be the first time I was mistaken.

So with the car running you should see a 3-6v between the pos battery terminal and the I terminal (and 6-9v I to ground)? I need to check this as soon as the car is on the road again.