I've just repaired the wiper delay units on both my 3500 and 2000TC. Both can be removed from the wiper motor in situ, with a modicum of fiddliness. Both had the same fault and I'd presume most P6s would be the same. On the side opposite the plunger is a one way air valve. The spring for the valve is a piece of sponge foam which of course stops being springy with time. Once you get the unit out, repair is a doddle.