parrot 3100 mazda 3 bose/nav

I'm considering installing a Parrot CK3100 bluetooth kit in my 2008.5 Mazdaspeed 3 with Bose and OEM nav.

If I didn't have the Bose stereo, it would be a matter of a $20 harness and plug and play installation. Bose looks like it requires a $130+ kit. I'm thinking about manually hardwiring it. I think the problem with the Bose systems is the Parrot audio signal is inserted before the amp so it gets amplified and ends up being way too loud. There may be a way to attenuate the signal at that point, but I assume I could avoid this by tapping into the speaker wires after the amp. It may just mean running some longer wires.

Does anyone know if the factory head unit has a lead for mute? I assume it does somewhere because it can be done from the steering wheel controls. Can anybody tell me where I can tap into it? I know the other option is to utilize the remote turn on lead for the amp, but I'd much rather mute than turn the amp on and off.

My other question is if I'll have issues with the voice directions from the nav unit. Does anyone know how the nav is connected to the speakers? I believe it is independent of the HU since I think people have replaced the stock HU and the nav still works over the speakers. Does the nav have its own audio amp that taps into the speaker wires somewhere (similar to the Parrot unit)? If so, does anyone know where this is? I don't think I want the nav directions to cut into my phone calls. Surely someone who has done an aftermarket audio install has discovered this if they ran new speaker wires to comps in the doors and lost the nav voice. Is it possible that the nav audio signal is inserted before the bose amp? Where do I need to tap to insert Parrot audio after the nav audio?

(Not sure if it is relevant to this issue, but I am also adding an aftermarket amp and speakers if it makes a difference)

Any info is appreciated!