It's been raining and that spot is wet again. I think the roof guy
will warranty his work but I might have to find a way to fix it myself
because I'm in CA and it doesn't rain very often. My roof is about 8
years old and it's got a layer of old roof below it already. It could
be leaking down from some other spot but I would think it's likely the
valley..

Any advice is appreciated.

Wait until everything dries, and ppour a bucket of water
into the valley. If it ends up inside, then that's where the leak is.

Drill a 4" hole in the lowest part of that valley, and install
an internal drainpipe. This is why only people with more
"artistic" sense than common sense build roofs that way.