Where is the automatic transmission relay control located

Auto Zone diagnosed my check engine light as having an A/T Relay Control Issue. I have ordered the part from Kia, is this an easy at home repair? A/T remains in 3rd gear (fail safe) I know from reading and researching. I do not know where the relay control is located, although I have been told it's under the dash at the end of the heater box? Is this correct?