Second fan not coming on when I turn the AC on

If you are referring to the second speed of the inside fan blower motor, (heat/Ac). The most likely the problem is the resistor pack. This twenty-thirty dollar part has various stages of resistance to speed up the fan as the the switch is turned up. Do not buy a used one - it is not worth the headache. Go to a parts store and get a jobber part to save a lot of money. If it is the electric fan on the radiator, then check the temperature sensor switch, (usually mounted in the radiator). When the sensor reaches a certain temperature, it closes the circuit and turns the radiator fans on. You can test these with a multimeter a the plug when it is connected and running. If this is normal, check power coming in with the plugs disconnected and ignition switch turned to the accessory position. Lastly, you can turn off the ignition, set a jumper between the plug not connected to the sensor. If you switch the ignition on and the fan does not work it could mean you need a fan motor. Almost always it is the sensor though. If you only have one fan, do not run it till hot to test the sensor. Instead remove the sensor from the radiator, hook up the multimeter to both leads in the plug. Set the meter to Ohms. It should read zero. Put sensor into a small pot of water being careful to keep the plug away from the water. Bring up the temperature of the water and if you get a reading when hot the sensor should be okay.

Run a hot wire to the 2nd radiator fan to see if the motor will work.

Easier to just buy a rad fan switch and screw it i after checking rad fan as FOR suggested. Nice tutorial, tm. You got my vote!