One more thought ..
don't expect to see voltage coming out of the Light Swithc unless the
key is on.
Engine does not have to running, just key on.
Be Sure you don't make that mistake ....there's no power to the light
switch unless the key is on.
< the Dimmer/Flasher switch has full time power though ) .

also ..
always keep a known-good or new Ignition Switch in the van to just try
quickly.
All you have to do is drop the lower steering column cover, unplug the
connector at the back of your Igntion switch, plug the new one quickly,
and turn it with a screw driver.
that's always worth doing as...
the effort/possible fix or useful info Ratio is quite high.  It's easy
to do, might fix it or move things forward. Any small easy test like
that is always worth doing. And every van should have a spare Ign Sw in
the on-board back-up parts stash.

Scott
www.turbovans.com