Technic is making a plastic speaker adaptor now.
It looks like he can now sell you most of what you need for
a hook up kit in one fell swoop.
Also I saw this but its from the UK
http://www.ebay.co.uk/itm/BMW-4-FRON...-/190740622344
It can take awhile to get a PM reply from him and he doesn't like to answer if its on the weekend
I guess he actually uses his weekends for something besides this. So he will probably answer in a few days.