Final answer:
Connect the yellow wire of the 4-way flat connector to the: left turn/brake light on a trailer.
Step-by-step explanation:
When connecting a yellow wire of a 4-way flat connector, it is typically connected to the left turn/brake light on a trailer.
This is a common wiring configuration used in trailer connections. The yellow wire carries the signal for the left turn and brake lights, indicating to other drivers when the vehicle is turning left or braking.
It's important to consult the wiring diagram or instructions specific to your trailer and connector to ensure proper connection.