We have had two leaks on our C30. One was when it was new, over the port side in cabin. Apparently the port nav light was not sealed adequately. We didn't know for sure, but it looked like that light was unsealed, so we sealed it from the outside, and it stopped leaking.
Another was more recently, over the helm. I considered pulling out the head liner to see where it was coming from (the most definitive approach). But first I just resealed everything on top with Lifeseal. I thought it was time anyway, after 2 years. It seems to have stopped, and my guess from visual inspection is that it was from the horn connection. If it recurs, I will remove the headliner and look for trails.
In short, inspecting and resealing everything on top might stop it. Both of ours were relatively obvious and that worked.
If not, I'd suggest to pull the headliner. You might also find it with a moderately high pressure hose application to suspected points. (I don't really like the idea of adding to the leak, but OTOH it is important to find it.)
Another idea is to use differently colored water at suspected ingress points. (That is a last resort for me in such a location because I don't want to permanently discolor anything that shows.) Now that I think about it, I suppose there may be clever solutions such as clear but UV visible liquids.