There are many ways to do this. Cheap to expensive.
Let's start with cheap. You can plug your earphones right into the Sporster's line out. Volume control is problematic, so you will want to set the Sportster's internal volume to the max value (it's in the menus someplace, since I have the same unit). Then you will need to go to Radio Shack and get an inline volume control (about $5). This will allow you to control volume without having to page through menus on the Sportster. If the max volume isn't loud enough, you will need to find something like a Boostaroo to help amplify the sound.
Next gets more expensive. You can go with an intercom system, like Autocom, Instaride, Baehr, or other system. Expect to spend upwards of $300 to get this working. Or, like me, you'll spend close to $500-600 to get everything hooked up -- radar detector, satellite radio, ipod, FRS radio, GPS, ...
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'06 K1200GT Crystal Grey
'04 Sprint RS Caspian Blue
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