Before the dealer bashing starts, the question I have is: When they came to install the radio, did they know it was to be interfaced with the intercom, and was a price provided to include that with the install? If so, then they ought to either make it right or credit for the intercom interface.
If, however,like happens to me from time to time, they were not told ahead of time and possibly only an installer was sent and had no way to make the correct changes in the radio, then you need to pay for that additional visit,and get it taken care of.
By the time you buy software and cable, its likey gonna be $$ wise better to let them handle it.
of course there is alway two sides to every story.