I'm not sure about a softail but the touring bikes have an unused switched power wire and ground behind the headlight. Also an accessory plug under the seat. You can hook the GPS directly to the battery, but I have mine switched to the ignition and I don't have to remember to turn it off, when I stop. If you search around this and other forums, you may get a more specific answer. I know this has been asked a lot of times.
Silly me, I figured it out on my own, since at that time I hadn't read on the forums much.
:USA