Shimano GRX RX 815 Series Dealer's Manual page 45

INSTALLATION/REMOVAL
Handling electric wires
3.
Open the grommet from the center, and place it over the electric wire
in the appropriate position.
Perform the procedure after determining the length of the electric wire to expose from
the frame in advance.
4.
Install the grommet.
(1) Insert the grommet so that it hooks onto the wiring hole of the frame.
(2) Push in the grommet with your finger.
Installation example to the down tube
(1)
Junction [A] side
(2)
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