Double Radio Antenna Connection

[bigwalts4runner]

Today was getting ready to install my Pioneer AVIC-X920BT in my 4th Gen 4Runner, when I noticed that there are two antenna connections, and the Aftermarket only has one Antenna Connection. I connected the larger Antenna connection to the Pioneer and the FM sounded a bit distored. Do both antenna connections have to be connected for the FM/AM to work? Why are there two Antenna connections?

 

[JeremyM]

See - http://www.toyota-4runner.org/audio-electronics/70806-aftermarket-head-units-2.html#post611260