Thanks for the replies. It seems that most folks travel with a 2nd “just in case” light and I intend to do that as well, and the solar one mentioned above caught my eye - I didn’t know there was such a product but now I wonder why it isn’t more common.
On my Iceland tour, one of my taillights died from corrosion, rain got into the switch.
If solar ones produced much light I am sure they would be more common.