Wipers not stopping (intermittent issue)

Folks,
Thanks in advance for any advice here.
09 Mondeo, 1.8 TDCi Hatchback.
Of late (last week or so) when I drive with the wipers on, sometimes, the wipers do not stop, even when the key is removed and/or engine is off. Nothing will stop the wipers until: I have found that if I open the engine bay and tap/knock on the fuse box under the bonnet the wipers stop in the position they are in.
I can go again in the car and find that if I just "feather" the wipers (manually have them on every now and again, I don't have an issue, but generally if I leave them in the first (slowest) full on position, they will remain on in some but not all occassions, I suspect it's down to the length of time I have them on.

Sorry for the overly confusing post.
I am thinking/hoping that first place to start might be to replace the relay once I figure where it is in the fusebox and where to get one.

Any opinions much appreciated.