Bonneville Switched Power For Satnav?

Hi I got myself a couple of piggy back spade connectors and ran it off the horn on my tiger seems to be working ok

 

Under the seat is a useless place for the USB connector imo but its assembly can be unplugged from the bike harness, so what I did was to unplug it, cut the cable then extend it to run under the tank and up to the instruments. The USB socket is now on a homemade bracket behind the fly screen, handy for the (in my case) iPhone holder.
Photos here - https://www.thetriumphforum.com/threads/phone-holder-and-usb-move.9154/#post-107934

 

Hi, I connected my sat nav to front side light in head light unit

 

Re-using this thread rather than start a new one:
Just picked up my 2017 T120 and bought a new TomTom 550 for it. I had assumed one should run wiring under the tank to the seat and use some feed off the fuse box. My question was, how does one get the cables under the tank? without taking it off!!
But, if its ok to run the satnav off the horn 12v feed that sounds easier I guess? Anything to take note of in order to access the horn i.e. undoing the assembly on the handlebar etc?