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Brights don't work at all right?
You stated low beams operate only in certain positions.....does this van have DRL's? Does it have "auto" lamps?
Does your headlamp switch have a position on them labeled "AUTO"?
DRL is daytime running lamps.
ok, so just to clarify here the low beam Headlamps (and all the rear parking lamps...be sure of this) operte when in the second position then correct?
Have you tried replacing the headlamp bulbs yet?
I sometimes run into some of the craziest situations like this because the filaments in the headlamps short out and cause backfeeds in the circuits. $30 in bulbs to save $400 in complex electrical diagnostic charges is $$$ well spend for the at home do it yourselfer even if the don't fix the issue.
Replace both headlamp bulbs and retest this issue. If it does not resolve the issue (and it may well do so) then the system needs to be checked with a meter and a schematic, the headlamp switch may be internally shorted here.