Final answer:
The brown wire of a 4-way flat connector is connected to the tail lights and side marker lights on a trailer, adhering to standard vehicle and trailer wiring color codes.
Step-by-step explanation:
The brown wire of the 4-way flat connector is always connected to the tail lights and side marker lights on the trailer. This color-coding is part of a standard used for vehicle and trailer wiring. The brown wire is typically designated for running lights, including the tail lights and side markers, which are essential for visibility during night and under low-light conditions. When wiring a trailer, it is important to adhere to the color codes to ensure that all lights function properly and safely.