Don’t know the answer, but things I have found I will share.
1./ 99% of all electrical problems come down to bad contact in the connectors. Undo all the connectors behind the panel, and at the engine relay box, clean them all, spray them with switch cleaner, and put them back together.
2./ The Volvo panel has a number of diodes in there, so it’s not just a case of simple lights in there.However, this is all potted, so I doubt any water has got in there. It could be the ignition barrel (connections again), so give it a good spray with switch cleaner, and see if that cures it.
You can get switch cleaner from RS components. While you’re at it, get a tin of IPA (Isoprpol Alchohol), which is also excellent for cleaning and degreasing, and a tin of PTFE spray (which is excellent for lubricating stuff).