Hi Paul,
this one is quite common on GP's. I'm not convinced however that it is down to the aircon drain pipe alone.
Aircon on systems have a "dryer" which is a catch tank of sorts that is supposed to catch and collect water from the aircon system. These can fill up over time and do occasionally contribute towards the issue you mention. That's one.
Two, I have the same issue with mine, I was told that if my windows were spotless inside (clean of any grease/dirt deposit) then condensation would not occur. Well, I can vouch that after spending significant amount of time at the beginning of each winter to clean the windows there is some truth to that. Basically, no matter what you clean windows with it will always leave a micron thick film on the glass, the ticker the film the more condensation/bigger droplets, the finer the film the smaller the droplets. After using all methods under the sun, I find the best way to clean is when the window is misty, the wiping the water with a clean industrial paper roll absorbs the water and leaves fine streaks of dirt on the paper. The trick is to always use clean paper to wipe with, so after one or two wipes, the paper should be rotated. Granted it uses allot of paper, but once you have been through this procedure once or twice, you will notice that the windows mist will be finer and finer and clear up much quicker. Smokers, good luck with the (ask me how I know). That's two.
The other thing is that Aircon is a dehumidifier when working properly, so it does dry out the air inside. When using the aircon I find that switching it off (the aircon not the fan) a few miles or a few minutes before you park up helps towards not having the aircon system release the captured moisture back into the cabin. That's three.
Happy to listen to others opinions and views on this as it is a common complaint if you look around, but that's how I handle mine (dryer yet to be replaced on my 07 GP Multijet).
Humour