Wiring the car for towing is fairly easy. There are a number of good wiring kits available and these should be used, rather than trying to design your own. All trailers will need brake lights and turn signals as well as tail lights and side marker lights. Most states require back-up lights for trailers as well as extra marker lights for overly wide trailers. Some trailers are also equipped electric brakes. Others can be fitted with them as an option, depending on the weight to be carried. Add to this an accessory wire to operate trailer internal equipment or to charge the trailer's battery, and you can have as many as seven wires in the harness.
Determine the equipment on your trailer and buy the wiring kit necessary. The kit should contain all the wires needed, plus a plug adapter set which includes the female plug, mounted on the bumper or hitch, and the male plug to be wired into the trailer harness. When installing the kit, follow the manufacturer's instructions. The color coding of the wires is usually standard throughout the industry.
One point to note: some domestic vehicles, and most imported vehicles, have separate turn signals at the rear. On most domestic vehicles, the brake lights and rear turn signals operate with the same bulb. For those vehicles with separate turn signals, you can purchase an isolation unit so that the brake lights won't blink whenever the turn signals are operated. You can also go to your local electronics supply house and buy four diodes to wire in series with the brake and turn signal bulbs. Diodes will isolate the brake and turn signals. The choice is yours. The isolation units are simple and quick to install, but far more expensive than the diodes. The diodes, however, require more work to install properly, since they require the cutting of each bulb's wire and soldering the diode into place.
The best wiring kits are those with a spring loaded cover on the vehicle mounted socket. This cover prevents dirt and moisture from corroding the terminals. Never let the vehicle socket hang loosely. Always mount it securely to the bumper or hitch. If you don't get a connector with a cover, at least put a piece of tape over the end of the connector when not in use. Most trailer lighting failures can be traced to corroded connectors and/or poor ground connections.