I see that we already discussed the idea in this thread:
http://empegbbs.com/ubbthreads.php/topics/367442/1

... Including the idea of simply resistance-mixing your bluetooth with the empeg's line-level outputs to the amplifier. You correctly stated in that thread that you'd want the satnav to be able to interrupt the music playback in order to tell you to turn left. Even resistance-mixing the outputs together, so that both the satnav and the empeg made sound at the same time, you'd need the volume of the satnav to be loud enough to be heard over the empeg when it's blasting, and soft enough to be non-startling when the empeg is quiet.

So yeah, the "satnav interrupter" is what you need.

The problem is, I don't know if the Bluetooth specification even has anything in there about the satnav voice instructions interrupting the audio. I don't know if that's even possible to do in Bluetooth. I don't recall seeing it in the spec while I was working on the BGE, but maybe I missed it.