On newer cars, the constant 12-volt and the ignition wire connections can be made through the factory wire harness. On older cars, the fuse block can serve as the constant 12-volt source. Ground the radio to the chassis. Connect the antenna to the proper antenna input on the radio. Connect the radio to the amplifier so when you turn on the radio, you are also turning on the amplifier.
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.