First, I cannot believe we are fighting about this topic with other members. Gang, keep in mind, the Monte and Impala are W-Body cars. Yes, there are some differences, but 90% of the car's make up is the same. Matter of fact, I just found my wife's '05 Impala has a defroster issue and probably the same cause BBEngineer posted.
Now, in my quest (that is not yet complete, as I still have to FIX the problem), I heard of some GM cars that do not use an antenna mast having a radio/defrost module that goes bad. I did suspect that with the Monte, but I was having a REAL pain finding information about it. Nobody seemed to have a part number for the item, no one seemed to list it. Tons of peeps with defrost issues, but none listed there solutions (if they ever found it).
I applaud BBEngineer, he stepped forward with some info that really helped me bring my research to the next level! He gave me a name of someone that could get me the part number to the module (and the guy did, he also said there are two different part #'s GM used in my year, part numbers 10289183 and 10399641, you have to remove yours to see which you need). Those numbers cross ref to Monte AND Impala.
Knowing this info helped me make better use of an online GM shop manual site, the '04 Monte manual and Impala manual are the same and here's a link documenting replacing the module:
The problem is, that shop manual site lists that module as a fail point for bad radio reception, but I could not find ANYTHING that mantioned it as a fail point in the Defroster circuit. But, as noted, that manual lists replacing that module.
When I get the car out for summer driving, at some point this summer, I will be looking into this problem further, thanks to the info BBEngineer provided me.
There may be mid-generation changes, maybe some of the 6th gen Montes don't have this module, I don't know. All I do know is I now have part numbers and steps that document where and how to replace this module.
Admittedly, since I have not yet fixed my problem, I can't say BBEngineer is 100% right, but I am confident that to the best of his abilities, he has provided me the best info he can to help lead me to a resolution. THAT is what we're all here for.