89 B350 - no headlights, no park lights, no dash lights.
Incoming voltage (2 different red wires) are both only half-illuminating the test light.
(switch is removed)

Interior dome light function is all that will work using the headlight switch. (clicked all the way over to the left)

Have turn signals , radio illumination and radio functions.

If i knew the "half-illumination" was correct , i would just buy a new headlight switch.
But ........ idunno whether that is the right reading - or a low reading (meaning something else is wrong)

If i knew which terminals to jump on the plug - i could try that.
Tried Mitchell-on-Demand online but the wiring diagram page won't open for me.

Didn't use van for about 2 weeks - cranked it up the other night just to give it some exercise - no headlights or park lights - all of a sudden. No previous symptoms or problems noticed before with them at all.

EDIT : high beams are activated by turn signal stalk , not a floor mounted button switch. fwiw

 

Check fuses. They may allow enough current to pass for a tiny little bulb, but, not for the headlights and such.

 

Replaced fuses on suggestion and no improvement.
After several disconnect/re-connect with the switch -- now it won't operate the interior dome lights anymore.
It -had been- still working before.
But the dome lights will still come on when a door is opened.

Its probably the switch itself dying/dead.

If anyone can come up with which terminals on the plug to jumper-wire , that could prove or disprove the switch.
Will take about a week to get a switch delivered .
Sure would hate to just twittle thumbs for a week , then turn out the new switch makes no change. ugh

 

Might be able to find wiring diagrams for it here.

 

That is the site i was trying to use before.
Maybe i need to sign up on the site to be able to drill into pages further.

Found a "pin-out" for a Dodge headlight switch on here via search.
And it matches my switch terminals.



And a wiring diagram for a 92 Van - which is probably "close" to mine


Was going to do jumper wires last night but voltage at B1-B2 had dropped to about 1v or 2v
Probably why the dome light had stopped working
They were around maybe 6v-8v before.

Have made some discoveries, could get voltage back at 12v on B1-B2 with some wiggling and jiggling.
Seemed to be this "relay/breaker 13-14-15-16" needed a unhook-rehook exercise.
Maybe coincidental

To try to cut through all the drills and trials for now
When i push the brake pedal - B1 & B2 both lose power - wild {with headlight switch disconnected}
Going to take the bulbs out of tail lights and see what happens next. On the next trial.
And then disconnect brake light switch at the pedal - as step #2

When i would get 12v back at B1-B2 and hook up headlight switch , nothing worked.
Unhook headlight switch and B1-B2 = low voltage again.
i -think- maybe whatever is going on with brake light system is tripping that "relay/breaker 13-14-15-16" as soon as i plug in the headlight switch, and it has to recover.

Electrical is my weakest ability

I didn't realize (test/check) before that i did not have brake lights either.
No Headlights
No Park Lights
No Brake Lights
No Dash Lights
No Flash-to-Pass HighBeam
so it is un-driveable even in the daytime without brake lights - sshhttt

 

Also just realized the Horn doesn't work either.
AND - mashing the Horn also kills the power to B1 & B2 on the headlight switch plug.
do what ?

So ... hitting the Brake Pedal or the Horn - kills the power to B1 & B2
It recovers power slowly , not always 12v - sometimes like 6v or 8v

 

Ok, I don't know if this will help, but recently I replaced my headlight switch, and then the turn signals didn't work. So wiggled and pushed that same relay you showed in firmly, because I read that relay will fail and cause that issue after a few decades 🤣.

I did not pull it out all the way to inspect it, because as you have found they are hard to locate and replace quickly and I didn't want to break it. But after I did that, the turn signals worked. On my van, that relay is in the fuse panel on the side of the dash, and the horn and backup light fuses are right next to it. And of course the headlight switch is right next to the back of the fuse panel. I am wondering if that is the same for your van.

When I first touched that relay, and the headlight switch before it right after finding the turn signal didn't work, they were both hot to the touch while the other fuses were not. I would not be surprised if my relay is also going out and it is slowly affecting the components with fuses closest to it, maybe even damaging the wiring nearby and the fuse panel itself. Have you checked around there, smelled anything burning, etc? Something to consider in addition to the wiring in general, since crap rolls downhill.

I will be interested to know what happens when you put the new relay in when you get it.

 

I think our vans are probably about completely different on wiring arrangements and whatnots.
Headlight switch is on far left of dash (push&pull old timey headlight switch)
The relay/breaker is in behind the instrument cluster about middle of dash.
Fuse box is under the glovebox lid.

I had to remove dash fascia and instrument cluster to get to the back of headlight switch to disconnect it - and remove headlight switch in general. The relay/breaker would have been hidden behind instrument cluster.

Nothing getting hot or warm or smelling - relay/breaker seems fine so far.
Still have turn signals and wipers (wipers are part of turn signal stalk)

Its going to be something wacky as he// - more than likely
Going to unplug brake switch - and then both horns after while.
I don't understand how the horn is getting into the mix ... over my head right now.

 

Unplugging horns and brake switch had no effect.
The relay i mentioned is called a "multi-use relay" #4373307 - described as possible uses :: AC Clutch relay, Speaker relay, Load Leveling relay ...... so it's probably not the piece in the wiring diagram. (2 pink wires/1 black/1 brown)
Whatever it goes to - it only has the same low voltage / fluctuating voltage going to it as B1 & B2 on the headlight switch.

I've got a deep issue somewhere down in the deep, that is effecting a whole supply string.
Probably will be 2 more days before i can do anything else.
Planning to pull the battery , to get access to the bulkhead wiring block connector that mounts to the firewall on driver side by the steering column, and see if there is corrosion inside of there.

Another REALLY ODD thing that happened right at the beginning of all this - on the first start after 2 weeks - the wipers came on when the key was turned to RUN position. But the wiper switch was Completely OFF.
Took a few times of re-keying and messing with wiper switch to get them to Stop.
I guess that was my sign - hey buddy i have a Surprise for You
From there i started finding out all these other things had stopped working.
But the wipers still work (pfft) until i disconnected them today. (no change)

Wipers - Squirters - Turn Sigs - Radio all still work.
So they must be on a completely different supply line.

 

You have a bad connection somewhere in the power circuit. Need to trace that one back, and check EVERY connection it has. One of 'em is bad.