Hi Lynsay, I see nobody has offered you any help...
I think that you need to buy an ISO interfaces for aftermarket ICE to work with the HK amp and speaker system. It's called an HK Enabler and is found on NewMiniStuff.com website in the "1st gen Mini & cabrio - Interfaces, fitting kits etc" section... (sorry can't post links to other sites)!
Also talk to Mickey the Mini the owner of NewMiniStuff.com, he is a very helpful and knowledgable guy.
Hope you get it sorted - Andy