Appropriate BJT for car circuitry
I don't get why the transistor is necessary, just use a couple of diodes:







simulate this circuit &ndash; Schematic created using CircuitLab

When the low beam wire is on, only the low beam LEDs light up, when the hi beam wire is on, all LEDs light up (D4 is there to prevent backfeeding to the car system - my car would not care about that, but a newer car might).