Since I'm in no rush I'm taking the cautious route here and am asking you kind ladies and gents to confirm that I've found the correct plug under the tank to use as the GPS feed. I'm 99% certain it is, but what the heck! It's blowing a gale outside and hailstones are falling almost horizontally outside the window! Gives me something to do!
Here's a couple of photos:
Was cable-tied to the frame tube immediately behind my thumb.
Plug showing blanking over.
Blanking cover removed...
The supplied wiring loom that came with the Garmin GPS looks as if it is going to fit reasonably well after a bit of hacking, slashing, soldering and cable tying!