Do not connect any of the amplifier channels together. The chips do protect themselves from shorts across and shorts to battery and ground, but connected to each other is more risky. Each of the six speaker output chips are rated 45 Watts into 2 Ohms, at the chip. There are some losses in the amplifier assembly so real world power is a bit less. This is just shy of 10 VAC sine wave. Aftermarket speakers need more voltage. It is not worth the effort to connect an aftermarket speaker to the OEM amplifier.