I have been having this intermittent wiper fault for some time now, and decided to try and locate the cause.
I did suspect the wiper relay, so this was my first port of call.
As I did not have a spare, pulled the relay, carefully lifted the cover, then one of the tangs broke holding the cover !!!.
With the aid a strong light and magnifying glass, located a very fine hairline crack on the circuit board to one of the external connectors.
Soldered over the offending crack and as a belt and braces approach, also added a link wire to the circuit.
Result, all functioning again, pics below.
Before
After
