It sounds like they are suggesting its the CEM that's got wet.
suffered water ingress on my XC70 2005 and the water was actually dripping out of the bottom of it, and a lot more problems than you are describing.
you don't need to cut the wiper assembly to get it out but it is a bit of a bugger to do.
have a look at this video it gives you a detailed guide to removing the CEM
https://www.youtube.com/watch?v=U8UozG9qLvM
if you are handy with the tools its some thing you can do yourself, I did mine and after cleaning and drying everything out it worked without a problem.
if you do need more work to get it sorted I would talk to Lukasz at volvo diagnostic
https://www.volvoforums.org.uk/showthread.php?t=143579