Kevin H. Patterson designed this solution for trailer light wiring after installing a towing hitch on his vehicle:
Custom Brake Light Controller Expands your Trailer’s Capabilities
This is a power module designed to control trailer lights based on signals from your vehicle’s lighting circuits. Most vehicles have at least 4 separate circuits: Running (Tail) Lights, Brake, Left Turn, and Right Turn. Most basic trailers have a 4-wire connector with only 3 signals: Running (Tail) Lights, Left Turn, and Right Turn. The trailer does not have a separate circuit for Brake lights; applying the Brake is supposed to light up both the Left and Right Turn signals together.
The board can be purchased on Tindie:
Creltek 3-Line Trailer Light Converter
4-Line to 3-Line Combining Tail Light Power Module for 12V Systems