It might depend on the context (some native speakers mix up idioms, so you might run into examples where someone has used "shine" in place of "shed"), but in general, to "shine a light on something" is more like to call attention to it or emphasize it. You could also say "to highlight" something. To "shed a light on something" suggests that something was previously obscure, but it is now clear (as though it was in the dark, but you have turned on a light). To "shine a light on something" suggests that something was not very noticeable, but you have brought attention to it, as if by shining a light specifically on it. It was not necessarily /dark/ before, but you have made it brighter so that it will be noticed.

Never seen it with "a" in there. Shed light on something = common expression

Davidk, thank you for always giving me perfect answers.

Hmm. I would say “shed light on it” or “shed some light on it” NOT “shed a light on it”. It means to reveal something that was obscured “Shining a light on it” means to highlight or call attention to it. Let’s hear what the eyewitness says. That may shed some light on what happened here. This ridiculous situation has been going on for years. It’s time to shine a light on it.

They basically mean the same thing, only one is past tense and the other is present. Both are free to use in a similar context.