I think a have figured a way you can tell if it is the Ignition Switch or not.
The (+) voltage goes through the Ignition Switch when the Key is in the start position and goes to the Starter lock out/Back up light switch on the side of the transmission. From there by way of a VI wire (Purple) it eventually splits with one Purple wire going to that Terminal block (to activate the Starter Solenoid and Crank the starter) while another Purple wire before it reaches that Terminal block has split of and goes to the Glow Plug Relay (this is the wire that keeps the Glow Plugs on while the Starter is cranking).
There should be a Purple wire in the small square Connector on the picture Glow Plug Relay
Here is where the fun starts.
Disconnect the small square Connector on the Glow Plug Relay (this is to bypass and make sure that a bad Glow Plug relay is somehow supplying (+) voltage to the Purple wire).
Next; with the Ignition Key in the Off position Connect your test light to the far right side of the Terminal Block where the Purple wire screws in. If your test light lights and your Ignition Key is in the off position your Ignition switch is the problem. (It cannot be the Starter lock out/Back up light switch because that switch gets the power from the Ignition Switch.)
If you do the above test (remember the Ignition Key is still in the off position) and
your Test Light does not light up; plug in the Small Square Glow Plug Relay Connector. If you your Test Light lights up when the Connector is plug in it means you have a short circuit in your Glow Plug Relay and it is giving (+) voltage to the Purple wire when it is not supposed to.
I think the above is easier to do than it is to write instructions for.