It ought to be not far from starter itself as the piggy back solenoid takes a fair current. Trace the thinner wire from this solenoid (should be red/white) and it will take you to the relay.

Richard

2018 Roadster 3.7
1966 Land Rover S2a 88
2024 Royal Enfield Guerrilla 450
1945 Guzzi Airone