This article is from the Fishing bass FAQ, by Bass Rogue with numerous contributions by others.
If the problem at the vehicle jack is the test light never comes on,
then the problem is probably a broken ground wire or a bad ground
connection. Trace the white wire from the jack back to where it
connects to the vehicle frame. If it doesn't connect to the vehicle
frame, then connect it there. A good connection requires a crimp
termination (round lug) on the end of the wire, a metal screw or a
bolt and a self-locking nut, and a star washer. The frame connection
point should be bare shiny metal. To test this connection point, turn
on the vehicle's running lights and connect the test bulb between this
point on the frame and the brown wire pin on the vehicle jack. The
light should come on. If this connection point is good, then check
for an open or broken ground wire.
If the test light fails to come on at one pin, then you probably have a open
wire going to the jack. Retrace the wire back to it's connect point and ensure
the wire is not broken. Also check to see if it has a clean, mechanical sound
connection that is protected from the weather elements. If in doubt, redo the
If the test light comes on at too many pins on the vehicle's jack,
then the wires are probably shorted together somewhere. Retrace the
wires back to their connection points and look for places where two or
more wires are pinched together. If the wires are good, make sure the
connection points are not shorted. If the connections are physically
right next to each other, separate them a couple of inches to avoid
the potential for a short circuit.
If the test light comes on at the wrong pin, then the wires are
probably connected to the wrong connection points. Retrace the wires
to the connection points and reconnect them correctly.
One of the things to keep in mind about tow vehicles is they are not
all a like. Some vehicles, especially the ones with factory installed
tow packages, have a trailer pig-tail wrapped up in the tail light
wire bundle. If you find a place where the wire bundle appears to be
too big, then you have probably located the pig-tail. Undo the wire
bundle and you will probably find a coiled wire bundle that doesn't go
anywhere. That wire bundle is your trailer pig-tail. All you need to
do is connect a jack to the wires. Just follow the instructions in
your vehicle user manual.
Another thing to keep in mind is some vehicles use five wires instead
of four. These vehicles use different bulbs, usually amber, for the
turn signals. In other words, the brake and turn signals are
different light bulbs. The five wires are:
Left turn signal
Right turn signal
In order to go from these five wires to the four wires discussed
about, an adapter is needed. These adapters can be found at any
automotive store. Some are universal ones that connect directly to
the wires; others are designed for specific vehicles and plug into the
wire harness at certain points. Keep in mind, these devices can fail,
they have diode logic circuits inside of them.