Hola!!
Puedes intentar instalarte el Garmin Communicator Plugin desde aquí:
http://www8.garmin.com/products/communicator/
Una vez lo tengas instalado conecta el GPS y enciéndelo y ahora en Google Maps busca el botón de "Enviar"...
Te debería dejar enviarlo directamente al GPS.
Haz pruebas a ver si así consigues lo que quieres ;-)
En cuanto al firmware del Zumo 660 aquí tienes una lista de cambios desde la versión 3.30 hasta la actual 3.90:
Change History
Changes made from version 3.80 to 3.90:
* Corrected spontaneous shutdowns that could occur when satellite reception is temporarily lost.
* Corrected intermittent issue with music not resuming after a navigation prompt when paired to A2DP headphones.
* Corrected some languages not showing up on the unit.
* Corrected problems with the turn arrow in More Map View.
* Corrected a shutdown that could occur while reviewing off road tracks.
* Corrected an issue that could cause the unit to not shut off when connected to external power.
* Corrected importing only the first 650 points of some off road tracks.
* Corrected incorrect color on Netherlands Topographic Map.
* Corrected an issue calling some POIs.
* Corrected a possible shutdown when searching for cities.
* Corrected unit freezing when reviewing some POIs.
* Reduced the volume of the proximity alarm tone when using a headset.
Changes made from version 3.50 to 3.80:
* Corrected a possible freeze while playing MP3s while paired to a helmet and a phone.
* Corrected to allowed routes to be imported from matching maps on MapSource without recalculation.
* Corrected an issue that could cause a failure to transfer phonebooks from a Bluetooth connected phone with phonebook transfer support.
* Corrected missing keys on Turkish keyboard.
* Corrected an issue that could cause the unit to freeze when connected to a BMW mount and using the page key.
Changes made from version 3.30 to 3.50:
* Added TracBack.
* Added Autozoom toggle setting.
* Added Off setting for Proximity Alerts.
* Added dynamic range compression for louder voice guidance.
* Connect the headset before the phone on startup.
* Draw Arabic map labels in the correct direction.
* Fixed several route import issues.
* Fixed calculator percentage function.
* Fixed selection of Arabic, Chinese, Japanese, Korean, and Taiwanese text.
* Fixed some Bluetooth pairing issues.
* Fixed map pattern drawing.
* Fixed editing of the current route.
Un saludo.