Just this weekend I was looking at a nice leftover? 09 Classic. I asked about adding rear speakers to it like the Ultra and almost fainted when I saw the H-D price. $600 byt the time you added in the wiring harness ( I can only imagine how much that is per foot). Also found that in order to be covered under the factory warranty, it MUST be installed by the dealer...didn't ask how much that would cost, was still in shock.
Hog Tunes does sell a set of rear speakers also, but it looks like they come with an amp?...not sure why....and they aren't much cheaper.