Wires going into 2007 factory radio?

[j73m6]

I am replacing the factory radio in my 2007 Explorer with an Alpine gps navigation system.
There are two connections going into the factory radio, and I know what one of them does (the larger one). Any idea what the smaller one is for? Do I need it?

I bought a wire harness from Crutchfield, but that seems to be only for the larger connection.

Thanks!