[Jeff]

Sid, Your theory is sound.
In both my houses Mesquite leaves blocked water flow in the flashed channels that are either in the valleys or where a lower roof meets an upper level wall.
When it cools down, pull up just enough tiles to gain access to the flashed channel. Clean out all debris. Rinse & Repeat. On both my houses the boards running horizontal, to hold the tiles on, were too long and stuck out into the channel catching debris and starting the blockage. Trim those back.

Do you have attic access above the outlet box? If not, you can remove the box and take picture with your phone to see if the water is coming from the direction you think it is.

My car gets dripped on by the garage door if it is opened during or shortly after a rain.

If you need a roofing guy text me. My guy is cheep and good, but hard to get a hold of if the crappie are biting.