To run my three external gauges (Oil pressure, AFR and boost), I've just been using the "Add a fuse/circuit" things that plug into the interior fuse box. Then I've been grounding the gauges to an already existing ground that is behind the dash.
I don't have any issues thus far (I plan on running my AFR gauge ground into the ECU once my new one arrives).
Now I'm just curious if any has come up with a better/cleaner solution to this.
I was thinking I could run an extra circuit from the battery, through the rear shelf, down the floor (under the carpet) and then up to the dash to a distribution fuse block for the gauges.

 

My fuel pumps run off of 10 gauge fused wires from the battery. So does amp and power inverter. My big fans up front pull off the alternator lead, and a few other things in the engine bay do this as well. For in the cabin, I'd probably just pull offer the battery with fused power. I also use factory ground points.