If you find the FM radio performance lacking, don’t bother ordering a signal performance booster, as the OEM antenna already has an integrated powered amp. As others have noted, you should check to insure that the antenna mast is snugly screwed into the antenna mount. You probably shouldn’t get a ‘stubby’ antenna either, as your radio performance may suffer some more.
Ideally, a FM antenna length should always be 31 inches, and that is why you’ll notice that a wire is spiraling along the shaft of the shorter 16″ OEM antenna to replicate that. You might not like the look, but if you are really out in the sticks, you might actually try a ‘true’ 31″ antenna.
If a FM station offers HD, you’ve probably noticed that the sound quality is best when HD is displayed. When the digital signal strength is inadequate, you should notice as the radio switches back to the regular analog signal.
The FM antenna also serves as your AM radio antenna, but your GPS Navigation antenna (behind your instrument cluster) and your XM radio antenna (sharkfin on your trunk lid) are separate.
Regarding the popular aftermarket 5″ rubber ‘stubby’ antennas, they too have a 31″ wire coiled internally around a much shorter shaft, but they will never pull in stations as well as OEM or true 31″ antennas, but if you spend most of your time near a large city, you may find the performance to be fine. This ‘stubby’ that is also marketed for the Mustang fits very nicely on the 124 Spider, as shown ⇓.