Electrical systems...
Not being an electrician, I was a bit nervous about tackling this part of the project. We contacted a couple of real electricians - but again found that they wanted a lot more money than we were prepared to part with.
So I bought a "Black and Decker" Basic Electricity handbook down at the Home Depot and started at it. Turns out most of the basic wiring is pretty straightforward (why they wanted $6,000 for a couple day's work is still beyond me...).
We used Romex wiring, and wired the place like any conventional house. The main difference is that in a typical house, you staple the wire to the wood framing (for support).
Here we stapled the wire to the tires and buried the wires under the mud as the walls were built out.
Some places may require all the wire be run in conduit, if so, just run the conduit in the grooves between tire levels.
My bravery went only so far however - so we did hire a licensed electrician to install the mains service to the circuit breaker box. I also had him check over all the wiring I had done. There were a couple minor mistakes that he charged $60 to fix (a far cry from $6,000). So our part of the project was simply to run the interior wires, switches and outlets and connect them to the breakers.
We haven't gotten to this stage yet, but ultimately we will harness the energy of the sun (and wind) and convert our entire Earthship to solar/wind (the real "off -the-grid" experience).