Direct Current, such as used on a car, trailer, or anything with a battery, only uses 2 wires to complete a circuit. You have your positive post & negative post on a battery. The negative connects to the chassis of a car, and the positive is distributed throughout the vehicle through fuses, switches, relays, etc. If you took a DC light bulb and connected it from the positive terminal of the battery to the negative terminal, it would light. There you have created a simple DC circuit. Every circuit in the vehicle is based on that.
When you connect a trailer to the hitch of your car, the tongue & hitch make a ground connection, but this should never be relied upon. The minimum amount of wires needed is 4, as the previous poster said; Running lights, LH turn, RH turn, & Ground (the LH & RH are used for stoplamps also). If you have four separate light bulbs in the back, one on each side will be used for running lights, the others will be used for their respective turn/brake lights. If you only have 2 bulbs, they are dual filament, and can only be installed in one direction.
This may be more information than you need, but when it's how you make a living, sometimes it's fun to share.