I do not see any reason that it will not work with the 550 or any GPS with an external stereo output port. The kit does not mute the radio's output it just lowers the volume of the radio when a turn direction signal is present. This being the case you would not be able to MP3 or XM via the GPS, which is why they include the cable to connect the GPS to the AUX input. At least that is how I see it working, without the kit this is just my quess.
OK, Jim - I am now in possession of the kit. I've emailed you the instructions and wiring diagram for your perusal, and I'd be interested in your comments as to whether you think it possible to make it work with the Bluetooth kit....
The module part itself is potted, so can't see what's in there.
One interesting thing, though: it says that it's for the Zumo 665, but "can be used with a 660 after a software <i.e. firmware> update to 3.70 or later." As the output from the Zumo is simple stereo audio via the jack, it's hard to comprehend what difference the firmware rev of the Zumo can make, and indeed why it's restricted to the 660/665 at all.... (Mine is a 550...)
Look forward to your comments once you've seen the wiring diagram, Jim.
Cheers,
Jim