Hello & Welcome to Just Answer,
Sorry I didn't see your question earlier and to hear of any issue you are having with your device.
To help you with this issue, may I ask:
1) Do you have another (different model) of GPS that you have tried to pair the helmet to (successfully) - such as a motorcycle specific model (Zumo)?
I only ask because the Nuvi models generally are intended to pair to a cell phone only, and the Nuvi itself behaves and a Bluetooth headset (or hands-free speaker-mic).
So in effect, what you would be doing when trying to pair the Nuvi with the Bluetooth headset in the helmet, is to pair TWO "headsets" directly to each other. That is why the Nuvi is reporting the "incompatible device" message you are seeing.
Just so you know, the motor-cycle specific GPS units often DO support helmet headsets, to be able to communicate with the device, hearing spoken directions, etc. So if that is what you are looking to do with a GPS and this helmet, you may want to take a look at the Zumo line of devices.
I hope this helps clear up why you are seeing this behavior. Please let me know if you have any more questions.
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Thanks & Travel Safe, Greg A.