I have a 95 Volvo 850 Turbo Wagon. I've had a very intermittent problem where, when I use my front washers, the wipers and washer pump will not shut off. Sometimes this clears itself in some unknown way, other times it does not. What seems to trigger it is holding the wiper stalk in the washer position for more than a second or so but this happens so infrequently that it is hard to tell.

When this first happened over a year ago I thought that I had diagnosed it down to the wiper controls on the column so I replaced this unit. The problem went away. Then, sometime later, maybe 1-3 months (?) the problem reoccurred, so I then replaced the wiper relay. The problem went away, although at some times the washer would stay on much longer than normal after releasing the stick but it always cleared itself. I seemed to be able to avoid the problem by simply pulsing the washer control as opposed to holding it.

Well now the problem is back and seems "stuck". Any good way to diagnose this? Since I've replaced the other two major components in the system, could this be something to do with the wiper motor? Or wiring?