Wiring your car for trailer lights is not the most exciting job but with a little patience and instruction it can be easily done. The best way to get this job done is to make sure you have the trailer, along with the car, that you plan on hooking together.
You first need to look at the trailer that you are planning on using to pull with the car. Determine what kind of trailer plug-in is installed on the trailer, a flat 4-prong plug or the round receptacle style? Once you know which style is on the trailer you can now purchase what you need to install on the car.
You can actually install either style plug-in on the car but now you will know to buy an adapter to fit the one on the trailer if needed. If you need to wire the trailer the kit usually comes with both ends, one for the car and one for the trailer.
Typically most cars use the flat 4-prong plug-in, the round style is more likely to be put on trucks that pull a large horse trailer or a 5th-wheel that uses an electric brake. So we will stick with installing the flat plug-in trailer connector.
If you have a newer vehicle, and by that I mean late 80's, early 90's and newer, then you may be in luck. Vehicle manufacturers and after-market companies now have wiring connectors that you can just plug into the existing wiring system on the vehicle. All you have to do is unplug the wiring on the car at the connectors leading to the rear lights and install the trailer harness connector in-line with the car's wiring system. The trailer harness connectors will match what is on the car, as long as you purchase the correct harness for your vehicle. If this is available then it's the best way to go because it's very simple and easy to do.
If you aren't able to just plug in a new harness then here is what you need to do to get that flat trailer connector on your car and ready to go. You will want to use the female half of the connector (1 metal terminal exposed) for the car. Decide where you want the connector. Some people put it in the trunk to keep it out of the weather when not in use, but I prefer to put it outside and keep it taped or tie strapped to the hitch.
The white wire on the connector will need to be attached to a good ground such as a bolt or screw running through the metal bumper or frame. You will need to determine which wires are for the left turn/stop light, right turn/stop light and the taillights on the car. To do this you will need a simple 12-volt tester with a light. Just turn on each light one at a time and test the wires for the correct circuit.
Attach the yellow wire on the trailer harness to the left turn signal and stop light wiring. Some will use what are called scotch locks to quickly make the connection at the wires, but I like to solder and heat shrink the connections if possible. Attach the green wire on the trailer harness to the right turn signal and stop light wiring.
Then attach the brown wire from the trailer harness to the tail light circuit wiring. Finally check your work by turning on the lights and using your 12-volt tester to make sure all connections are correct at the end of the plug to make sure you don't blow a fuse (or melt the wires) when you hook up to the trailer end of the harness.
If you have to install the harness connector on the trailer then just make sure the wiring is done the same way as above with the same color-coding.
It's really nice when you can purchase the trailer connector that just plugs into the existing harness. This makes for a clean connection and you don't have to splice the wiring. But if you have to go the old fashioned way don't let it scare you as it's not too hard to install the universal flat connector once you get the wires figured out.