Most radios have a power antenna output. This is a 12-volt line that runs the power antenna. Use the power antenna lead to also turn on the amplifier. Use a relay to make the connection, so that the line can run both the power antenna and the amplifier turn-on. Ask a neighborhood car audio retail dealer how to connect the relay.
Some radios have a separate power antenna output and a separate amplifier output.
If this is the case with your radio, use the amplifier output to turn on the amplifier. The wires run from each speaker to the amplifier. The longer the run, the more resistance on the wire. The larger the gauge size of the wire, the less resistance. Run the speaker wire on the same side of the car as the signal wire.