sorry, that is actually a few things at once.

pull the neutral safety wire from the relay to prevent the starter from trying to start the van.

the switch has a plug on it, so you can do 1 of 2 things.
you should be able to stick the probe in the back of it, but if you can't, then pull the plug slightly so you can get a probe between the switch and the plug.
so the test will be to see between a ground not on the switch and the ign 1, ign 2, and the other post that goes out what the power drop is if any before you turn the key and after.
so I don't remember the posts that goes out to the bulk head.
but you have start and run on the ignition switch.
both have power going into them.
when you turn the key to start, and hold it, you should be able to test the power out of the switch. same when you leave it in run.

having the neutral safety disconnected prevents you from hanging the starter on the whole time.