Product review (5 stars): I couldn't be happier with the result
Air handler fan ran regardless of any switch position on my thermostat.  Turning off the circuit breaker was the only way to stop it.  After researching online, it seemed the relay switch was the probable cause. I took the cover off my air handler (4 screws), and then the cover over the relay switch (2 screws). The number printed on the original relay was the same as the one I was looking at on Amazon. I confirmed that it was the right part by entering my A/C model on the Trane website.

After receiving the relay, I labeled all the wires to the old switch, then replaced it with the new one.  It was very straight forward, and even though the new switch was slightly different, the connections all had the same labeling.  I turned the circuit breaker back on and the system worked normally.

I couldn't be happier with the result.  Saved a lot of money and had the satisfaction of fixing it myself. I have never done any heating or A/C work, but if you are moderately handy, and are not afraid to take the cover off your air handler, you should have no problem changing out the switch.