QUOTE(mtc911 @ Jun 3 2019, 12:02 PM)
Attached is what the setup looks like - Relay is under the car, which seems odd to me...
Oh wow, yeah...that's pretty odd. The yellow lead should plug right into the starter like so:
The terminal on the starter is the solenoid terminal and acts as a relay of sorts already, that external relay is just adding an extra point of failure. Some definite Department of Redundancy Department stuff going on right there, unless, somehow, the yellow wire is so high resistance that it can't properly trigger the solenoid in the starter, but can trigger the relay coil. (The wire comes from the key switch, to the under-seat relay, to the engine bay relay board, and then finally to the starter solenoid.)
My guess would be that water got into that relay. I'd try pulling the yellow wire's spade connector out of that relay, and hooking it into the spade connector on the starter and see if it starts with the wire hooked up there instead.