I was thinking of doing the same thing. I have an 03 FLTR and the radio sounds like crap once the bike is doing anything over 2mph.
I was going to patch into the speaker wires and even bought the female plug from radio shack. Then I started thinking about that switch. The manual simply says that it's not operable on the road glides, but it obviously is. The forward position turns the speakers off, the middle on, and I presume that if there were rear speakers, the backmost position would have both front and back on. If yours is the same, you might want to wire it so that forward position is the headphone jack. Then the speakers wouldn't have to be on as well.
I'll post up when I get mine wired, and let you know what I did.