Cleaning Cladding

[Moncha]

is it originating from any specific point like seams, drain points etc...?

[thebear54]

Like most people who wash their cars, I used to not wash all the seams and door interiors. I got the streaking too. Really bad when I owned black cars(which ain't never gonna happen again...black looks great,but a pain to keep clean).
The residu from road dirt and air pollution sound like the culprit here. So wash all of the the car, not just the surface areas and give those areas a good coat of wax. Just my 2 cents.
John