For fifth-wheels and other types of tow trailers, 12v power is supplied to your rig while on the road via what is called an umbilical cord (abbv. u/c). The cord plugs into the truck's female receptacle on one end. The other end is fans out to various exterior lights (stop, brake, turn) and also supplies power to your emergency brake control, if you have one.

It's important that you know how your umbilical cord is wired up to your trailer. You can probably go for quite a while without this knowledge, but eventually something's going to break. What would you do if you accidently pulled away from the rig without first unplugging the u/c? Now you have a mess. You probably think you can just join all the wires together, red to red, blue to blue, etc. Well, if your rig's anything like mine I can almost guarantee that the colors won't match up. So here we're going to prepare you for any u/c eventuality. After we're done you will know where all the external rig wiring goes and how they are connected to the u/c.

Before you begin, I recommend you make a drawing of your truck's 7-way receptacle, like the one shown above. It will likely be mounted on the rear bumper or inside the truck bed. On this drawing you will be showing the seven pins and the function of each. You don't have to number them. As you detect what each pin is used for, make a note. Keep the drawing with you in your rig.

First, let's find out what each pin on your u/c outlet does and also verify that the outlet obeys the RV standard shown above. If not, it may be SAE standard wiring, in which case the colors red and yellow will be swapped, as will green and brown. You'll need a multimeter, one which has at least a voltmeter and ohmmeter. A cheap one will do fine.

Turn off the truck ignition and unplug the u/c.

First, let's see which pin is ground. You might logically think it would be the center pin, but it probably isn't. For sockets obeying the RV standard it will be the white wire at the 7:00 o'clock position. Since the ground wire is connected to the frame of the truck, there won't be any current flowing between the chassis and the ground wire. So we're going to use the ohmmeter part of the multimeter. An ohmmeter measures the resistance in a wire, in ohms (who would have guessed?). When you touch the two meter leads together you will see the readout drop to zero, indicating a lack of resistance. When you touch one lead to the chassis and the other to the ground wire you should see the same thing, except maybe it won't go all the way to zero since the metal chassis does have some resistance, as does any metal.

If you find that the pin at the 7 o'clock socket position is in fact ground, then great. Otherwise something is amiss. Either the wire is broken or disconnected from the chassis, or ground is found on some other pin. Use your ohmmeter to check the other six pins to see which one is actually the ground. If you find it, make a note of the proper pin on your diagram.

A human interest story: When I did this it turned out my ground wire wasn't grounded at all ☹. It had shook loose on some very bumpy roads in Canada. Having a ground connection is critical for your u/c to function, so how did my u/c make a ground connection without the white wire connected? Apparently the whole grounding thing depended on the frame of my trailer resting on the fifth-wheel hitch to establish a mechanical ground connection between truck and trailer. That's scary! Guess God's been looking out for this old redneck boy after all.

Ok, back to work. Now that you've found the ground wire, let's use the voltmeter portion of the multimeter to verify the other pins. First, put the red (+) lead on the 1 o'clock pin and the black (-) lead on the ground pin. This is the "hot" lead so you should be getting a reading of 12v or so. If not, then you need to go searching the other pins for it. When you've found it, write the pin position on your diagram.

The hot lead is sometimes referred to as the "ext" lead. It is used to supply juice to your rig's external lights, like the turn signals, stop lights, flashers and running lights. It also maintains a charge on your house battery while driving.

Now that you've isolated the ground and hot wires, turn on the ignition and turn on the left turn signal. Use the voltmeter to determine which of the socket pins is fed by the turn signal. It should be the pin at the 9 o'clock position. Put the voltmeter's black lead to ground and verify that this is indeed the correct pin. Since the turn signal blinks on and off, the needle on the voltmeter will move back and forth accordingly, so leave the lead touching each pin for a few seconds. If this isn't the right pin, keep moving the red lead to different pins until you get a reading. When you find the correct pin, make a note of it on your diagram.

Do the same procedure for right turn signal (3 o'clock) and tail lights (11 o'clock). Note that the pins for the left and right turn signals should also control the emergency flashers.

To find the wire that controls the brake lights (probably at 5 o'clock) you'll have to have someone push on the truck brake pedal while you visit the remaining wires. This wire probably also controls the automatic brakes on your rig.

If your hookup is like mine, there will be no backup lights connection (center pin, yellow wire).

If not, it is possible to repurpose this wire for other uses, if desired; something like a rear camera comes to mind.

-- https://www.rvupgradestore.com/Choosing-the-right-connectors-for-your-trailer-wiring-a/330.htm
-- https://www.etrailer.com/Trailer-Breakaway-Kit/Tekonsha/2028.html
-- https://www.etrailer.com/faq-wiring.aspx

Ok, congratulations. You now know what each of the seven pins in your 7-way does, and you've made a diagram for future reference. The next step is to match each pin on the plug with its associated colored wire at the other end. With the u/c unplugged from the truck, use your ammeter to find these associations. Put one lead of the meter onto one of the plug pins and then put the other lead to each colored wire until you get a zero (or low) reading. Write down the pin-color match on your diagram. Do this for all seven pins. Note: when measuring resistances it doesn't matter which lead you place where, as there is no current flow.

Note that pin positions are the mirror image for plug and socket. For example, the socket pin at 9 o'clock corresponds to the pin at 3 o'clock on the plug (looking directly at the plug). Likewise, socket position 5 o'clock is plug position 7 o'clcok. Once you have determined the color codes for all functions, plug the umbilical cord back into the truck socket and repeat the above procedure, this time testing the voltages on each of the wires at the end of the umbilical cord. This serves as a sanity check to ensure you have the correct designations from the truck socket all the way back to the colored wires at the end of the u/c. Your diagram is almost complete.

The colored wires at the end of the u/c probably collect inside a protective metal box. Open up the box and find the ground (white) wire. You might have to run a wire from the truck's frame to make testing within reach. From then on it should be easy to determine the other wires purposes, by testing for voltages between the wires and the ground wire.

The final step in completing your diagram is to make a note of the wire colors leading from the u/c back to the rig. Unlike the u/c these colors aren't likely to conform to any standard.

Don't forget to put the cover back on the box when done.