About a month ago I bought a '92 3.0L Taurus that has been overheating
and I've noticed that when I turn on the AC, the temp gauge will sit
between the "O" and "R", even after operating the car at speeds of
65mph during morning and evening commutes. Using the vent setting and
any other combination leaves the gauge closer to the "A" or "L".

I'm just getting into doing my own work on my cars, but I know, from a
little research, that the AC unit turns the radiator fan on and it'll
always run as long as the AC is running, so it seems as if the fan
isn't coming on early enough to prevent the engine from getting too
hot.

What could cause the fan to not turn on before a certain temp?

Is it the thermostat?

Since I just recently bought this car I'm not really sure what type of
thermostat was used, but I'm thinking that I should replace it with a
t-stat that opens at a lower temp and hope that it'll fix the problem.

Is it a Fan switch or something?