This article is from the Fishing bass FAQ, by Bass Rogue with numerous contributions by others.
It's very difficult to get people to check the ground connections,
even though most trailer problems are probably caused by bad grounds.
Some of the strange symptoms of bad ground connections are things
like: the lights on the wrong side of the trailer come on; the lights
on one side are brighter; the lights are on, but they're dim.
Here's what's happening when these type of symptoms show up.
Some of the bulbs on the trailer have two filaments in them. One
filament is for the running lights, they other is for the turn/brake
lights. Each filament has a wire going to it. Both filaments use the
same return, the base case which is connected to the vehicle's frame
or ground. Under normal situations, current (hole current) flows from
the positive terminal of the battery, through the wire to the
filament, through the filament, through the base, through the ground,
through the vehicle frame back to the battery negative terminal.
Okay, what happens if the ground connection is bad. Well, current
(hole current) flows from the positive terminal of the battery,
through the wire to the filament, through the filament, through the
base. Okay, it can't go out the ground because the ground is bad.
So, what happens? Well, it goes back from the base through the other
filament, through the wire to the bulb on the other side of the
trailer, through that filament, through that base, through that
ground, through the vehicle frame back to the battery negative
terminal. When all this happens, the filaments won't have the full
voltage across them, so they will be dimmer, but on one side two
filaments could be lit and that should appear brighter unless the
other side filament is the brake filament. Okay, you get the picture.
When things start appearing to be real strange, start looking at those
In summary, trailer wiring problems can appear to be complicated.
However, a systematic approach based on eliminating things that are
proven good can help you focus in on the real problem. Also, when
things start really appearing strange, start checking ground