Nobody has mentioned the infamous R15, auxiliary fan resistor.
This connection fails fairly often. It's a big ceramic resistor waaaaaay down in the fender well, sorta under the front right side of the fuse box, under the AC hoses and other wires. It's sometimes hidden by the leaves that collect down there. Usually the wires get corroded by the heat it generates and they fall off. Can be a fire hazard also.
R15 only comes into play on the fan low speed, which is turned on by the AC pressure switch, then routed through relay K10, to the input side of R15. But, the wire to the fans coming from relay K9 for high speed is also hooked to the output side of R15, and it could be corroded off.
I had to replace one last year.
Do a search under Auxiliary fan resistor for the exact location and wiring details.

Also, to check the actual fans, just find the wire plug right near the AC accumulator, and unplug the connector; then put 12V on the wire and both fans should run on high.

DG