You can hardware your Garmin Handheld or nuvi to your vehicle with the appropriate Garmin USB bare wire power cable from GPS City. This video will show you the two best options for powering your unit. The cables will take the vehicle power and convert it to the 5V that your USB based device needs. http://www.gpscity.com/search/q=010-11131 Connect with GPS City; https://twitter.com/GPSCity http://www.facebook.com/GPSCity 1-866-GPS-CITY
I made this Interface cable for my Garmin eTrex H GPS receiver. It will work with other models. It saved me about £25 and only took about 1 hour to make. I can now update the software, import and export waypoints and transfer routes to google earth.