For those that have installed an aftermarket radio or are thinking of doing so, there is one step that is necessary for proper AM/FM reception. The blue control wire from your head unit needs to be connected, even though we don't have a motorized antenna or (2012 ES only) external audio amplifier. This wire still serves the purpose of powering the signal amplifier built into the base of the stock antenna.
On the car's main harness, if looking at it while plugged into the radio, this is the third pin from the left-hand side of the car on the top row. It is a grey wire. When purchasing a harness to connect to an aftermarket radio, there should be a blue wire coming from that pin.
And yes, this will make a difference in the sound quality of any received stations. Even stations that come in pretty well should still see some improvement (mainly a boost in treble). Of the 6 preset stations I have, only 3 come in without the amplifier hooked up, and even they don't come in that clearly.