RE: Audio in/out for your head unit
When I wanted to add an MP3 player to my Pioneer head unit I checked and
that unit had a aux input port that could have originally been used for a CS
library type setup or now other things.  I found the adapter that plugs into
that port and terminates in little 3.5mm audio plug that goes into my MP3
player, the same as headphones.  I ran that up and out the ashtray.  The MP3
player plugs right in.  It took me longer to figure out how to configure the
head unit to play from that (getting the right settings on the front panel)
than it did for the physical install.  So you might check and see if such an
adapter will work for you.