I've got one of the older Garmin GPS V handhelds and I guess I've been using that (sheesh!) since 2002. I keep the thing plugged in to the multiport 12volt at all times. I use the stock power cord.
What I do is when I reach my destination, I detach the GPS from the little mount on the dash, still keep the GPS plugged in, but put the GPS in my lockably tuffy center console. Fortunately, the Tuffy has just enough room between the door and the side to allow one to snake a power cord up and lock the door on top of the cord without causing any wear to the power cord. I've been using the same power cord for 5 years - no cord wear and no theft. All fairly convenient and no messing with batteries for GPS, either