Shimano GRX RX 815 Series Dealer's Manual page 91

INSTALLATION/REMOVAL
Installing junction [B]
2.
Connect the electric wires to the E-TUBE port on junction [B].
Be sure to push it in firmly until you feel a click.
3.
Pull the seat tube side and chainstay side electric wires into the frame.
When the battery is an external type
E-TUBE port ×4
Electric wire
Electric wire for front
derailleur
Electric wire for rear
derailleur
Chainstay
Seat tube
91