My Escape is on all the time. I love it when sitting and waiting or something.
You can buy aftermarket power ports, and then run it to something like the dome light, which always has power and then run a ground wire. Get some clips so you can just clip it in when you need it for visual appeal, or spend the time to wire it down the headliner and under the dash. For my Escape that is what I did, and it works fine at keeping a GPS charged. However, if it's drawing power and the dome light is on with the car off it's quite dim, but useable. To do it safely, I think if you put an inline fuse you'd be perfectly fine, and grounding it to whatever the dome light is grounded to.
