Have you had a look inside the powered Mount if you can find stereo L R connector in there.
The reason I ask is that there is a 3760 for BMW that connects to the Car using a special powered Mount and plays both speakers through the radio.
That might be the only way to get Stereo out of the nuvi by somehow simulating the BMW powered Mount Kit.
Now that leaves us wondering of what is that USB to Audio Connector For .
Also can you please explain how you opened the nuvi.
If you buy some more Johny Walker please post pictures.