A/C and Heat Temperature Sensor Location

My cabin temperature control works intermittently.  Sometimes the
temperature control works just fine, but when it doesn't, it goes to max
cold.

When it is not working, the fan ramps slowly to its max output while
blowing out ice cold air, regardless of the temperature setting.  Even if I
have the temperature set at 85 degrees, the fan will be close to max
output with cold air. If I bump it up to the next level, "Hot", it will blow
out hot air at the max fan setting.  If I bump it back down to 85, it goes
back to max fan and cold air.

To control this, I have to manually ramp the fan down to its minimum
setting, and even then the temperature of the air is very cold.

So it appears to be a thermostat control issue, maybe feedback of the
from the cabin temperature sensor.  

Am I on the right track and where is this sensor located?

Thanks..