Ok, so I got around to applying this today.
As hard as I tried, I couldn't get it down the center vents. I wanted to do it the proper way (take out the blower and spray through there), but as soon as I started removing the footwell aircon duct, I realized that this isn't something I want to do (too many pieces to remove, stuff could break, etc.).
In the end, I sprayed the whole can through the driver's side air vent. I could get the tube completely in this way. After I emptied the can, I checked the interor for leaks, and sure enough, the drain tube was leaking onto the carpet of the centre console. Oh well I thought... with frustration. I got a fan, dried the carpet. My word of advice: put a piece of paper between the plastic of the center console and the carpet. This will soak up some of the liquid, should it leak.
I then took the 20 minute break, as per instructions, but nothing was coming out of the drain tube at the bottom of the car. Oh bummer I thought, this is going to end up in my interior. My frustration and hopelessness increased.
Anyway, I had some lunch. I came back to the car about an hour later, checked for leaks, and the center tunnel was no longer wet, which I thought was good. I then started the car, opened the doors for a minute, and went for a drive.
Now, as I was pulling out from my yard, I noticed a steady trail of water (big droplets, almost continuous). I drove the car, stopping and checking fluid from the drain pipe. I drove and drove, and it was just leaking and leaking fluid, you could see droplets pouring down as if they were coming from a sprinkler or such. 2 hours later, when I came home (albeit with the air con on) there was still water coming from the drain pipes.
I also tried setting the fans to max with the button, this also caused water to gush out of the drain tube.
Lastly I dried the evaporator (temperature 22C, fan on, just compressor off). I couldn't see much water coming out after.
So, where was all this water comming from? I'm guessing most of it was the Klima Cleaner, and the rest was condensation from the air con. It wasn't a particularly hot day today, and we've had some rain, but to be honest, I don't remember the car leaking this much water even when it was new...
I'll post back in a few days in terms of smell/effectivity and amounts of water leakage.