There are several threads on 12volts in the forum. Just be sure to change the date to "anydate versus 6 months"
I've had my 172 for several years so I can advise you on your fuse box just in case they have changed model. You just need to check yourself. That being said, I had two empty slots, and ran one under the bath/shower to next to the bed, and ran another under the floor to above the kitchen sink (used a cable puller and followed the existing wires). (I couldn't tell you right now what guage wire or anything like that, I'm the person the bugs the home depot or lowes guy and let them give me the right stuff.
Good Luck
Lee