Yeah for the negative just earth it to the chassis somewhere. You'll probably find a few existing easy to get to ones behind the dash. Just look for a thread somewhere coming from a piece of metal that has one or more brown wires bolted to it. A quick check with the multimeter taking positive from your selected fuse wire and negative to your selected earth point is worthwhile if you're not 100% sure, as long as it reads 12v with the ignition on you should be good to go.
With regards to the fuse, thinking about it again, if the cigarette lighter plug on the power lead for your device already has a fuse inside the end of it that protects the device itself (i,e somewhere around a 6.5amp fuse), you can take your feed from the positive side of the horn or foglights fuse. This will mean the fuse in the fusebox wont need uprating.
If it doesn't have its own fuse in it the the best thing to do would be to still splice into the feed from the positive side of the horn/foglight fuse, but get a fuseholder with a 7 to 10amp fuse in and use that on the positive wire of your cigarette lighter socket. Place it as close as you can to the point where you splice into the feed from the fusebox.
Doing it either of these ways ensures both your device and the horn/foglights are protected by their own fuse, without the need to change the fuse in the fusebox.