Well, if the 2nd wire on the speedo eventually goes to the ecu it must just be so that the ground is guarenteed good. Because I grounded out the 2nd wire and the speedo works now.
I looked a little harder at the Fuel pump wiring and it looks like the green-blue should be the wire that shows fuel level. The red-black looks to be the fuel-low lamp signal. The only problem is I can't seem to make it work. IIRC, Nissans used a 80ohm-10ohm for empty-full signal. Those numbers may not be perfect, but they should be close. That's the gauge type I got for it, and the needle will lift off of empty, but even after filling the tank it shows about 1/8 full. The other wires coming off the fuel pump are the ground (black) and the power for the fuel pump... Those are the only two I'm sure on.
The stock Water temp gauge uses the signal from the one wire. It's basically the same theory as the two wire, except since it's just for a gauge they didn't need the precision of a grounding wire. i would think they would have the same resistances but god knows what the Nissan engineers did there. I haven't gotten out my ohmmetter yet... Mainly because this is the least needed gauge (I can pull water temps off my laptop hooked to my MSEFI).