Alternative HVAC solution ? or how to avoid fixing this the right way

Hello all you clever people out there !
I have this 2000 Impreza coupe that I managed to gunk up the HVAC system on. It's a long story, but basically, got the car cheap/used a few years ago, did some bodywork experimentation on while fixing rust decay and whatnot (I made a lot of dust), and later realized that it had no cabin air filter from the factory.
While attempting to install a filter (it involves cutting/Dremeling out a little cover under the dash and inserting a filter), I cracked the plastic housing around the fan or heater element (or both)...
Now, not only is the heating element radiator thingy and airbox matrix contaminated with tiny bits of carbon fiber, leaves, Bondo dust, and fiberglass, but it also leaks water into the passenger footwell when I run the air conditioner in the summertime :P
This is a 3rd car, so if I can only drive it when it's dry outside and warm enough to roll the windows down, that wouldn't be the worst thing in the world.... BUT, if I could somehow find a somewhat cheap and not overly complicated alternative way to keep the windows clear or keep the car warm inside if it's cold or rainy, that would be a plus.
My impression is that the cheap 12v dashboard mountable heated fans that you can buy are not particularly effective?

I have vacuumed out the car many times (seats, headliner, every nook and cranny, etc...), taken the dash apart to wash out and/or vacuum the ventilation ducts that I could get to, but there is still something in this system that gives me a worrisome cough after a day or two driving around in it. I don't want to get rid of the car, but I also don't want to give myself lung cancer or emphysema.
Replacing the entire HVAC system is a possibility, but I would rather not go through the trouble, since parts of it are pretty well buried underneath various metal beams inside the dash, and it's hard for me to twist myself around to even reach all the bolts (which is how I cracked plastic pieces in the first place)
I had considered things like hooking up a separate 12v fan of some type and piping it into the dash ducts (using something quick and dirty like perhaps a vacuum cleaner hose and some duct tape. But, air alone doesn't seem to keep the windows from fogging when it's really humid.

Soooooo, any ideas ?? :)

I probably won't tackle the project until the weather warms up a bit, but thought I would get some ideas ahead of time if you have any. Thanks !

(Then once that gets sorted, perhaps I can find an alternative solution to replacing the gas tank that leaks when it's above 1/2 full ;) )

It's a matter of a little practice, but pulling the whole dash isn't that bad. You can then pull the whole heater box assembly and replace it. The core likely has the dust and such trapped in it. Short term you could use a 12v defroster, and there are several options between 20 and 50 bucks if you google them.

When I had the plastic portion of the dash out last time, I remember stopping when it looked like I needed to unbolt and remove the metal structure that the radio screws into (after I pulled out the console, radio and HVAC controls of course), and also the one that spans the dash left to right above or below the steering column...
and then to get the core out there were the coolant lines passing through the firewall (which I don't have experience with)...
So, at that point I thought that there perhaps was an easier, less perfect solution that would be good enough.