The box that decodes the signal and sends it to the speakers is called "a radio."
Can one get an amplifier and the appropriate harnessing for the $80 it costs to get a factory Radio10 on this forum (from me, for instance) or through eBay?
The speakers go to the radio harness which plugs directly into the factory radio. The aux jack also plugs into the radio through a different connector. Without the radio, nothing is connected.