Of course the trailer lights stop working - I have to drag it 300
miles the day after tomorrow!

Specifically, what has stopped working is the driver's side blinker
and brake light, which are on the same output of the flat-four plug
on the van. After chasing the problem in the trailer's harness for
longer than I shoud have, I finally figured out that the problem is in
the van, not the trailer.

First, let me make it clear that the blinkers and brake lights on the
van itself work fine. The problem exists somewhere in the wiring for
the flat-four plug.

The unloaded outputs at the flat-four sockets for the both trailer
lights and the passenger side blinker/brake read ~12 volts, loaded
they read ~11 volts. (I loaded it with a bare bulb, but I get the same
readings with the trailer attached)

However, the unloaded output at the flat-four for the driver's side
blinker/brake is ~7 volts, loaded it's zero. Once again, it was loaded
with a bare bulb, so the trailer has nothing to do with the issue.

So, I know the problem is in the van's harness or the perhaps
something with the pigtail, but I'm not sure what I should be looking
for. The 2 feet or so of the pigtail that is exposed looks fine. After
that it runs behind the trim into the bowels of the van. At first
light, I guess I'm going to have to start pulling panels off. I took a
quick look under the van (2004 Odessey) but it's late and dark, so
I'll start again in the morning.

What could cause the voltage at one output of the flat-four to be down
by half and what would cause it to drop to zero under the load of a
single trailer bulb?

Thanks for any suggestions.