67 Dart 270 reverse lights always on...help please

All of the below is IMHO: (Take the advise at your own risk)

You could wash it is soap and water as long as you blow the water out.

Probably could douse it in Denatured alcohol without any harm that would pull any water out.

WD40 shouldn't do any harm or be an issue down the road just make sure it has time to dry away before you put it back. ( I left my makita drill out one night and it got rained on, I remembered that WD40 had a reputation as being a water replant so I ran the drill up to speed and sprayed in the vents, FLAME shot out and I stopped!)