I just installed the compact Stebel horn on my ST1300 and it fits like a glove, so I thought I would share a few details on the install.
On the right side of the bike behind the mirror is a bracket used to support the mirror but it has an extra hole which is perfect for attaching the bolt that comes with the Stebel horn. The only modification required is drilling out the hole to 5/16" so the bolt supplied with the Stebel horn fits. The nice thing is that you can use the mounting bracket and bolts that came with the Stebel horn as it was intended.
The hardest part of the install is removing the plastic. Once the mirror and side panel have been removed you will need to remove the bolts attaching the front panel. You do not need to remove this panel completely, you only need about and inch or so in order to drill the 5/16" hole. Just be careful not to damage the plastic. The hole is only sightly smaller then 5/16" so the drilling is fairly simple.
I installed a relay and provided 12V/20Amp power to the horn and it is loud.
Look at the attached pictures for more details on the install.