Where do you plan on putting your Garmin? You can easily tap into 12v under the dash somewhere. I would mount your GPS, feed the wire under the dash as you you intent to have it, and then add a power port under the dash at that location.
Keep in mind that some GPS' use the power cord as the antenna for traffic and it needs to be exposed somewhat to work properly. You will have to test your model to be sure.
Was thinking of a proclip if they ever make a mount for the 2013 models. have the traffic thing on my model,(forget what model at the moment) but live in NYC and the traffic alert does not really help. Basically wanted to cleanup the armrest area. Is it possible to piggyback an outlet from the fuseblock?