1. [b]Seems to me the running lights should be dim, the brake lights should be bright.
2. I've checked the existing wiring. Here's what I have:
brown wire from four-wire harness attached to the only black wire that extends from the light housing. (The light offers only one black wire and one white wire.)
How does the bulb "know" whether to light the dim or bright element? How can I reverse this.
3. In a related issue, there are four marker lights on this trailer. They are all this type. Like the other lights, this one comes with two wires (one white, one black). When hooked up (white ground, black hot), only one of the bulbs will light. All four of these lights do this. I've checked the bulbs and they all work, but only half of the "outlets" actually work. What's up?
It sounds (to me) that you have stumbled upon the "Secret Code" of trailer lighting
I found this out several years ago when I was rebuilding a Compact Junior Trailer. The light fixture manufacturers leave out an important piece of information!
The vehicle manufacturers know this fact, but trailer owners who want to do their own work aren't privvy to this ommission and are stymied by it.
In a [b]4-pin flat trailer harness, the wires are color coded like this:
White = "Ground" (circuit return)
Brown = "Running Lights" (dim filament)
Yellow = "Left Turn/ Left Stop" (BRIGHT filament on the Left Side
Green = "Right Turn/ Right Stop" (BRIGHT filament on the Right Side
Here comes the [b]SECRET of the Secret Code:
The Light Fixture Manufacturers all ASSUME that the Light Fixture will be installed ON the Metal Frame of the trailer. [b]OR
They ASSUME that the Light Fixture will be installed on the Metal Skin of an Aluminum Trailer; this skin is assumed to be screwed onto the Metal Frame. Because the frame and skin of a metal trailer is ASSUMED to be GROUNDED, The manufacturers NEVER give you a separate Ground Wire!
(The light offers only one black wire and one white wire.)
These are BOTH "hot" wires! One is for the dim filament, and one is for the BRIGHT filament.
I have forgotten which is which.
Your statement (#1 in the quote above) is CORRECT.
Because Fiberglass Trailers' "skin" is non-conductive, the light fixture cannot be grounded by the mere act of screwing it or bolting it to the body of the trailer in the same way it works on a metal trailer.
[b]You have to run a separate grounding wire and connect it to the metal base of the light fixture!
Once you figure out which provided wire is for the BRIGHT circuit, and which provided wire is for the dim circuit, you'll wire thusly:
White = your added grounding wire to the fixture's metal base on BOTH sides
Brown = Whichever wire is the dim circuit on BOTH sides
Yellow = Whichever wire is the BRIGHT circuit on the LEFT Side ONLY
Green = Whichever wire is the BRIGHT circuit on the RIGHT Side ONLY.
When you turn on your Left Turn Signal, the Yellow circuit will be completed thru the flasher
When you turn on your Right Turn Signal, The Green circuit will be completed thru the flasher
When you step on the Brake Pedal, Both Yellow AND Green Circuits will be completed.
When you turn on the running lights, the Brown circuit will be completed.
The White wire is the return path to complete any of the above circuits.