Never heard that before. All accessories that come with automotive power point plugs run off 12V. If the device needs less that will be handled by the device's charger cord, not the car. You should have no problem wiring in a new receptacle straight to the fuse box.
Check the specs on your Garmin and see how much amperage it draws at full power. It's probably way way less than any of the existing fuses (if it was an actual cigarette lighter that would be a different story). Get a fusetap and a cheap 12V voltage detector (or go ahead and spring for a voltmeter - it comes in really handy). Then just test the fuses to see which one behaves the way you want it to (off with ignition off, on with ignition on), apply the fusetap, route the wires and mount the receptacle and you're done. You can also hide the receptacle in the console if you can get the Garmin charger cord into it also.