Thats not exactly true. The driver side fan can come on independant of the passenger fan using the A/C as its high speed controls that fan. I would test the passenger fan by running voltage to it. The power source for both he is correct on. So if 1 works the other has power but low speed fans are controlled by the engine temp sensor. Try jumpering the 2 wire sensor on the intake area and see if the fan comes on if it does maybe your sensor is bad.