The flasher relay is inside the BCM, so that won't be easy. Your '18 will have almost identical engine box wiring to a '16, so you can use the FSM here to locate the connections and wires. The turn signal connectors are E103 (R) and E105 (L), they're 3-pin gray connectors. They come out of the Engine Room harness,
not the Front End Module harness like the rest of the headlight connections. I can tell you from experience it's very easy for harness legs in that section of the front end to disappear underneath or around the #2 Fuse & Relay Box, so maybe that's why you aren't finding it. You can see the harness layout here on PG-47:
https://www.nicoclub.com/service-manual ... 6%2FPG.pdf