Yellow cable (BATT/B+) The yellow cable should be connected to the fuse of the constant power supply in the car fuse box. If you are looking for a constant power supply in the fuse box: Use a voltage tester to check the fuse to ensure it still have electricity even if the car engine is off. Red cable (ACC) Connect the red cable supply power only when the car engine starts. Testing method: The tester bulb will light when the engine is on and won't light when the engine is off. Black cable (GND) Connect this black cable (GND) to the metal part of the vehicle. Most cars have screws or metal parts next to the fuse box. Connect the black cable to the screw or metal part. (As in the picture below).