It can be trailer wiring headache with car using one filament light for headlight or turn signal indicator.
Brake lights is push button circuit going to each light separately. Brake circuit lamps are not shown.
Two diodes are 25 volt, 5 amp silicon diode. Resistors are 5 ohm, 10 watt. Connections for car and trailer ground is not shown.
Try switches at top. Left is turn signal, middle is headlight switch, right turn signal.
