'74 Duster Radio

If you connect both speakers to your radio you will not have the correct impedance. Your factory radio wants 8 ohms of impedance connecting 2 of them to your radio will result in an impedance of 4 ohms. The radio will not sound as good and it will be hard on the output transistor and may cause it to fail. If you connect it to only one of the two speakers then you might be okay. The reason I say might is because the connections to the cassette player also being on the speaker might also effect the impedance.