There is a thermistor mounted in the evaporator housing that senses the air temperature and passes that information onto the a/c amplifier which will stop & start the a/c compressor as required. Physically it will be located in the evaporator housing under the passenger side dashboard, either behind or below the glove box.
Please be aware that there are many factors that can impact how long the compressor runs - and can cause it to run continuously. In fact, anything that prevents the air temperature in the evaporator housing from getting low enough to turn it off - outdoor air temperature is the single biggest factor - on a hot day the compressor will run longer, the a/c controls also have an impact - if you're set to draw fresh air in, the unit pulls the warm air in from outside and the compressor will also run longer and quite possibly, continuously - make sure that when you slide the lever to close the air intake, the intake actually closes, sometimes the cables between the lever and the flap come loose.
