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krl, have you checked the RDI fuse? I'm bringing this thread back as I solved this same issue I had on my car. Both fans should turn on with AC on.
I experienced Fan2 (driver side fan) never getting enabled. Fan1 (passenger side fan) would go on with AC and when overheating. I swapped in spal fans w/ shroud since I thought Fan2 motor died. Fan1 worked for a while, and then stopped working with AC. Realized 2x 25A fuses were blown, RDI and CDS. RDI is Fan2 and CDS is Fan1.
Now both fans turn on with AC. It's odd the BRZ wiring schematic doesn't include these fuses in the diagram for the radiator fans, but includes the relays.