Firstly dont get carried away and confuze yourself. If a wire has +ve at any one position of the ignition switch, it is a power wire. The ign position will tell you if the wire is ign, acc or start +ve.
Most power wires will show up as -ve when they have no power, as it will feedback through whatever to earth.
Now think about what was in the enginebay;
wiper motor, windscreen washer?, starter, alternator and reg, air-con?, horn, dash warning lights, auto neutral start switch, reverse light switch etc.
I would think nissan would have kept a basic colour code? a quick look at a few different model datsun wiring diagrams will say yes or no and should give you an idea of what went where.
For the thermos, just connect an ammeter in series with the power supply and this will say how much current they draw.
I would think 1 would draw close to 25 - 30 amps.
As for the immobiliser. ? There are to many kinds to say this wire goes to this.