2009 Aveo temperature gauge not working, fixed

The car drove, and had code for not reaching temperature fast enough, the temperature gauge needle was down,
and never went up.
On 2009 Aveo, the temperature switch is located on the plastic housing behind the thermostat. The sensor faces to the back of the engine, and the connector faces upwards. The sensor is a thermistor, and it is easy to check if it is ok.
It should have a resistance somewhere between 100 to 1000 Ohm. The colder it is, the higher the resistance.
It is easy to check the rest of the circuit. Just insert 500 Ohm resistor in the sensor plug, and the display should move up.
In my case, the sensor was ok, but with the resistor inserted, needle was still down.
The way the circuit is made, bad ground connection will add to the resistance, and high resistance means low temperature. The grounds are under the battery, on the fender wall.
I have cleaned the grounds, coated with dielectric grease, added extra grounding.
I have also added an extra wire from battery negative to the car body. This fixed the problem.
I have a generic shop repair manual for Aveos from eBay, and it locates the sensor in some unknown position.
The electrical diagram was also for different model year.
Some people have reported also having bad thermostats (not in my case).
I have tested it by running the motor, and monitoring temperature differential on the 2 sides of the thermostat.
Initially the back was hot, and front cold (thermostat closed). Then both sides were hot (thermostat opened)
My thermostat was fine.
I am surprised, that with so many problems with the car, GM didn't make all the relevant information on the 2009 Aveo public. That would make life so much easier.

xtra ground wire is good idea, prolly why GM changed from plastic housing to aluminum for new OEM housing, better continuity. Will doing this release AC control from ECU and give me back control of my freon addiction? I'm gonna peak at service book tonite for prepping skills.