Rawze adrino setup

[Lonestar10]

You don't tie it in it would be stand alone turn the AC on by turning Arduino on when Arduino is off or on rest of the system would work independently.

a relay to turn fan circuit on/off

You put a relay controlled by Arduino to turn compressor on/off . And the evap temp probe, high pressure sensor, fan relay, and compressor relay would be tied into the Arduino.

Rest of the system fans/blend doors would still be controlled by trucks current system. But all AC components would be tied to Arduino.