Headlamp switch diagram from a Chiltons '89 to '91 repair manual. Has some differences from your '92 diagram.
BTW, I have a '91 B250 with 318 and your description sounds the same. Also, I have had 2 recent electrical issues with one being low voltage at the ac compressor clutch relay with voltage coming through a fusible link. I bypassed the link because they all seem to be behind the brake booster. AC started working fine. Just a thought to consider.

 

Thanks GBWard
i can't view posted pics on this website for some reason -
any chance you could send that picture to my email address ?
Would like to have it for reference

Thanks man

Didn't work with it any further today.
Maybe tomorrow will be a better day.

 

Thanks GBWard !
Got the email with diagram.
Much appreciated

 

GBWard your diagram has been very helpful - and wayyyyyyyyyyyy better than what i was trying to use (92).
It shows me how/why the Horn is also involved in this fiasco - which is a relief . (lowers possibility of multiple faults)

And it shows the Interior Lamp Defeat Switch (gray - mounted on headlamp switch) which mine has.
I am still getting a clean 12v at that source - but it gets juice from a different supply line via Fuse #8
So that is another relief , to know why that is still doing fine.

Should be able to get to the bottom of this, but didn't touch it today.
The problem is with wire L1 - and it is going to be ahead of the FuseBox since Terminal B2 (on Fuse #6) is getting the exact same wackiness as Terminal B1 that isn't fused in the Fuse Box.

 

Got into it today , as expected the problem was with corrosion in the bulkhead connector.
Mostly in the lower passenger side corner of the connector - which was exactly where L1 Red-White wire is positioned.

Going to have to make a creative repair on 2 wires of the bulkhead.
By-Passing the connector itself , and running them inside through the speedo-cable hole in firewall , spliced back directly to their partners with disconnect connectors.

Wire #1 was the trouble wire (L1 Red-Wht) - it broke apart at the exterior bulkhead almost immediately.
Wire #2 (Red-Pink) right next to it - was still reading fine before disassembly started - but it broke apart during inspection.
Glad it did so i can fix it now instead of later.

Will post some inspection/repair pics tomorrow or later.

Van sat for 8-10 years before i bought it, possibly in a worst-scenario position for the drainage underhood to run down the firewall on that side (just guessing- idunno) .
I have washed the engine bay with a garden hose and blew it dry with a leaf blower a couple times too, in the past couple months.
But the level of corrosion wasn't from just the past few months.
However, some water did come out of the connector when i separated it from the firewall.

This is an area that should probably be pro-actively inspected on all these older vans.
I had already thought about making a rain tray/deflector underneath the cowl to hood gap to move water run-off further away from wiring and other sensitive areas.
May try to see about some weatherstripping that might stop it from getting through completely.

They all came out of the factory with a rain tray/deflector over top of the battery.
I doubt any originals are left intact and they have been discontinued/NLA for a long time.

Thanks again to GBWard for the very helpful diagrams he sent me.