From googling, it seems as though the drainage at the back of the engine bay would be the cause, although I'm not 100% sure if that's right as it seems to leak when the a/c is running. While going around a right hand corner, my centre A/C vents also vent humidified air, as if it's taking in some water when turning right. My steering wheel is on the right hand side so when looking at the front of the car from the front of the car, the master brake cylinder is on the left and the battery is on the right. I've removed leaves and dirt as much as possible as I can see, but to really unclog it, do you need to take the battery out?

 

Yeah pop the battery out. It's not too bad there's just one (10mm I believe) bolt holding a little bracket in. Get the battery out and clean in there real well to make sure that is or isn't the issue.

 

I removed the glove box and pulled back the carpet to have a look at the a/c drain. I haven't removed the funnel yet but it doesn't seem to be leaking around the funnel, nor is the foam circle moist. But while in there, I can see the water leaking out a lot in behind in the head unit. I've uploaded some photos and a vid.




Video showing water running out:

 

Are you sure it's not a leak in the heater core? (i.e. leaking when the heat is on instead of the AC).

It's a common failure point and that video seems to be JUST in front of the core.

Since it's also winter now, more people would notice issues with their heat.

 

So I took the head unit out to get a better look and it seems to be leaking from the seal where the arrow is

 

I managed to fix it. I removed the funnel (One plastic nut, one plastic nut was missing, the other was impossible so I yanked it off). I stuck a coathanger up it to try and clear it out, a small leaf fell out, with a bit of water. Tested it again, no leak. Put it back together, now water runs out the car...