There have been a few businesses that have offered under mirror brackets for the ST1300. In 2018 there is one that is making a 3d printed bracket that works and doesn't need the mirror housing plastic to be cut. The gasket for the inside edge of the mirror is a little wrinkled but that's about it.
PSR-USA markets the bracket pair for $79. They fit the bill. I'm using "four" led 20 watt spot lamps that were a find on Greedbay. The LEDs are light and the brackets hold them just fine. For my installation, I wanted the ability to turn them off if I needed too. A standard 30 amp relay controls power courtesy of a push button switch on a small bracket. The lights come with bare wires. I purchased waterproof 2 pin connectors from where else but Greedbay. If the link is bad, then just search for "two pin waterproof power". Because they are LEDs the amount of current needed is less than 3 amps. I have a aux fuse box under the passenger seat that is active when the ignition key is on. 18 awg red/black wire provides power to an automotive relay socket which is then wired to the push button switch.
The only NON obvious thing in the installation is the use of a internal tooth star washer between the end of the bracket and the bike. It keeps the bracket "bit" into the mirror mount. Without it, you might have to really crank down on the provided bolt to stop the bracket from moving.
There are other pictures available on st-owners.net. The catch is to see them you need to register. It's free, only an email address is required.
Amazon sells a small latching push button switch that's IP67 rated as waterproof. It uses a half inch hole so it fits the bracket nicely. I used the switch to ground the cold side of a power relay that is mounted inside the dash.
It would have been great to use an illuminated switch but ya can't use what ya can't find. It doesn't really need to be lit up. If the button is engaged, it's flush with the edge so you know the lights are on just by feel.
