Ford Repair: climate control, 2003 grand marquis, grand marquis

Question
I have a 2003 grand marquis. The fan will only blow on high Whether I am in ac, vent, heat or whatever. I ran the diagnostic For eatc and no codes were returned. I think the fan resistor is bad. My real question is, where is the fan resistor and how much of a job is it to change it. I thank you in advance for any help you can give me.  

Answer
I found no TSBs or messages related to this. Looking at the diagnostic chart I suspect the blower speed controller is at fault. The testing is kinda extensive, requires a DVOM.

This controller is on the plenum, outside the firewall, below the heater hoses where they connect to the heater core. Not easy to get to, and you want to do this with the engine cold!