Hmmm.. 3 wires to your antenna sounds a little odd, can you get a picture of it. Should be a standard aftermarket stereo antenna plug to your nissan diversity antenna plug.
As for power to head unit, take out the headunit, and use a tester on the yellow and red wire. You should get power on the yellow constantly, and red on accessories.
If not, trace this back and check your accessory fuses.