Blower only working on the "HIGH" setting? Well my friend, your blower motor resistor has gone bad. "Oh my bajeezus, what do I do?! Is this a $400 part! Oh me, oh my!"... the answer is "No". It is a $17 part with tax.
First, go to your local ford service dept and order one.
Next, open the door to your car and locate the GLOVE COMPARTMENT. Look below the GLOVE COMPARTMENT. You will see it attached directly to the blower housing.
Refer to pic #1 and #2.
Now you must remove the 2 tiny bolts holding it in and disconnect it. Pull it out, now prepare for install. Plug the new one in, mount it back using the old bolts and there, my friend, is the end of the install. Now grab a beer because you just spent 3 minutes of your life. Enjoy the many settings of heat / AC.
Refer to pic #3 and #4 for what the new one looks like.
Seriously though, some people may wonder whats up so I just did a quick how to for whatever reason. :-)