Not sure if the water is coming from below and wood is sucking it up? Or should I assume it's from above? It tapers off at the top of the rot. Should I try to fill the space, or cut out a section and try to replace? It's tongue and groove locked together. Thanks for any and all advice....!

I ended up replacing three boards. They started and ended at a window, so were only 55" each. I found a spot above that is likely the source and will silicone it once the paint dries. Thanks

Unfortunately, you're not going to know where the origin is until you start tearing wood off. Start with the first layer, you might be lucky and not have to go much further.

The leak could be coming from above, 6 inches in, and flowing out to the end layer.

Leaking window/wood siding advice?