Managed to get some time last week to strip down the wiper parked sensor (slip rings) attached to the wiper motor. Unfortunately this unit looks typically 'Mercedes clean', however I bent the slip ring contacts such that they press down firmer on the ring, cleaned them and re-greased the whole unit.
Now the wiper has only failed once over a period of four days (in the rain) - but guess that means something still isn't quite right!
The K26 relay module is located adjacent to the ECU box and isn't too difficult to remove. To inspect the relays within, the RTV seal needs breaking - contacts on mine all look excellent!
Thought this may help someone else
