Active job

Leaking in corners of garage - need to identify cause & repair

In the corners of my garage nearest to the automatic garage door the wood has been retaining moisture from somewhere and degraded over the last 12 months to the state it is now (see pics). The cause may be wind-driven rain leaking in around the garage door, or the water may be creeping in from underneath as my carport is sloped down towards the garage. I don't know which or how to stop it so I'd really appreciate some help to figure out what's happening, how to stop it and repair the damage.