Could you have got the linkage on wrong?
You might find photos of wiper and linkage on ebay.
Incidentally, Ducato wiper motors have a thermal cut out switch which sometimes cuts out when it shouldn't. If you undo the 2 screws holding on the motor casing you can see it on the brush plate. You could short it out, just be aware the motor could then fuse or burn out if you switched the wipers on when they were stuck down by ice or snow.