The speaker wiring could be touching the vehicle’s chassis, causing it to be grounded (shorted).
If the car has an amplified speaker system such as Delco/Bose or Infinity you will need to purchase an adapter or will need to replace the speakers.
If your car has a common grounding speaker system (the speakers in the vehicle use a chassis ground or a common grounding point for the negative side of the speakers) you will need an adapter or will need to replace the speakers.
If your vehicle is using older speakers that are the incorrect impedance the radio will not work. Our radios work with 4 ohm speakers; speakers with less than 4 ohms impedance will cause the radio to shut down or may damage the internal amplifier of the radio.