Where is the Intake air temperature sensor located?

It is part of the MAF sensor. Mounted on the air filter housing. GM 15911983 Loosen the clamp and remove the air duct, unplug the connector and then remove the two screws that hold it to the housing. Reverse the process to install the replacement. Do you have a code indicating a problem? If you have a scan tool you can narrow down the problem if you look at the sensor data and do a couple simple tests. If the IAT is showing -40 degrees, it implies an open circuit. Check that the pins in the connector are okay, then take a simple piece of wire and short pins D and E together. Does the scan tool now read 302 degrees? If yes, the wiring to the sensor is good but the sensor is open. Does the scan tool read 302 before you unplug and drop to -40 when it's unplugged? The sensor has shorted internally.
What you don't want to see is either the temperature staying at -40 when you do the shorting test or is it staying at 302 when you unplug it. Both of those indicate a harness issue, either an open in the wires or a short.

The code is P0113.

Do you have access to a scan tool or reader that can see live data? If so that one is easy to test by testing as above. P0113 is generally either a bad sensor or bad connection causing a high resistance. That makes the computer think that the air is colder than it is when compared to the ambient air and coolant temperature after the engine has been off. So, it sets the code. Then you test that the low ref on pin E is less than 5 ohms (IE it should show a ground) Testing the rest of the circuit as above will verify the rest. If it all tests good except for the senor, you replace the MAF-IAT assembly.