On my 78 you only use the dark green wire. the two light green wires are there at the plug at the firewall, but thats where they end.
Dark green is compressor on signal. the negative wire on the compressors connector just goes to a bracket bolt nearby.
Both dark and light green wires ARE + in either case.
Run the dark green to the sanden's + terminal, and ground the two negatives to the engine somewhere close, as long as the sanden instructions don't say something different.