If the amp is stereo, you run the radio output to both amp inputs (L and R). This connection is in parallel, but the impedance arguments do not apply as the amp input is high impedance. The parallel wiring should be symmetrical, namely the parallel wires should use the same polarity at the amplifier input. The amp output has two sets of outputs (L and R), and each is wired to its own speaker.
If the amp is mono, then the radio output connects to the amp input, and two sets of wires go to the speakers. Here you can consider the parallel vs. serial connections based on matching the amp output impedance to the speaker impedance.
If you can choose the amp, go with stereo so you can balance the volume across speakers based on distance to each speaker.