Originally Posted by DBidault

I'd like to suggest measuring the heater voltage under load. Keep the sensor plugged in and measure the voltage across the two heater pins. If you have a high resistance in the wiring, having a load on the circuit will drop more voltage across the bad point and your meter reading (measured voltage) will be lower. If this is true, the next step that I'd do is measure pin 1 to a good ground. If the voltage is close to battery voltage then you know that the other wire or connections are suspect.
For reference, do this procedure to a known good sensor and you will know what you are looking for.
What is the measured voltage at the fuse, and across the battery? I haven't looked at the wiring diagram yet, but this will give a good start on diagnosing the wiring.