so it's raining at the moment and I decided to go and see if I could tell where the water ingress was coming from. With the door closed so the glass seals as it should, I took a video looking up through the door speaker aperture and this is what I saw. The video won't upload or link so I took a screen grab of it. You can see the water dropping straight onto the motor for the window, it seems to be coming from either the corner of the window closest to the wingmirror, or it's the wingmirror itself. Too hard to tell. I can't see any way of creating a better seal from the exterior weather strip, I've no doubt it's not capable of being totally water tight hence why there's drain holes in the bottom of the door. Just seems like a massive oversight to have the water drain straight onto the motor like that.
Think I might be onto something here. The weather strip on the outside of the window on my car appears to be fitted different to what I've found online. The strip that runs along the base of the window where it meets the triangular rubber seal near the wing mirror are overlapping the opposite way leaving a gap for water to easily run into. See attached pics. (Mines obviously the red car. )
Problem I have now is I can't move the window until my parts arrive tomorrow in order to readjust the seals.
Fitted the new switch with the old motor today, still not working. Plugged the new motor in, works perfectly.
You were absolutely right that the problem must lie within the motor. I've tried stripping the old motor down to investigate further, but the fixings for the harness section are secured with bolts that are rounded off during construction. If someone wants the old motor who has a decent grasp of electronics, they're welcome to it.