That sounds reasonable, just watch you don't overload any existing wires if you wire it into something else - the gps should draw negligable current saying that so shouldn't really overload anything anyway
I have just bought myself a cigarette lighter socket that has a rubber cap which leads onto a rubber seal behind - I got it from Maplin for something like £6 which I didn't think was too bad.
For the wiring in side, I'll be putting 17amp wire onto it, with a 20 amp switch, and there will be a 15 amp fuse in line with it.
I don't smoke, so there's no problems there, but I want to use it for things like portable search light, and my portable 12V coolbox
What I will probably end up doing is running a 25 amp wire to a connector block, and wiring everything off that, as opposed to running some individual stuff back to the battery on its own like it is now. I don't isolate the whole electric system with a switch, ever since my parents engine alternator blew up all the electrics on their boat when their isolator switch started playing up causing surges through the systems.