This may help, when i was a MOT tester about 22 years ago, i seem to remember that i had to pass a car that came in with a washing up liquid bottle operated by the driver sitting in the driving seat, through an open window, as i had read someting in the testers manual,about something like, as long as the screen can be washed from the drivers seat that wwas ok, but since then the MOT has gone mad, not much more to test but lots more money!!