So I wired the Garmin Zumo mount to my bike a few months ago and it's held up!
In the end I utilised the blanking plug (99 or 100 in the picture above) by removing the rubber bungs and buying some male pins with seals and wired them into the old plug.
6 x Spare MALE pins with 6 seals, HM & MT Sumitomo SEALED series
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