[G33KSQUAD]

iSimple makes a kit that sounds like a new version of what you already have called the transit link, which plays through a radio station
http://www.isimplesolutions.com/tranzitlink/

Could also go the route of isimple tranzit usb. It's a hard wired fm transmitter. When installed correctly it does the job of adding aux and includes a USB port for charging (no data transfer purely charging).
http://www.isimplesolutions.com/tranzit-usb.html

Being that you already have nav, Sirius, etc I can see why going with an aftermarket head unit may be a stretch, but that is a complete solution if you decide it's worth it to you. A good installer could seamlessly integrate the Boston acoustics system and increase every aspect of your audio/entertainment system.