Burt and the iman,
Regarding upgrading from the OEM horn, I yanked the tank off the '81 and located the horn relay; has a "53" on it in this photo. If memory serves, this relay was unused when I bought the bike a few years ago. PO had installed a fairing and other farkles that were removed by the time I got the R65, so this relay was likely used for an accessory.

Here's one way to
get loud:
Disconnect Negative (–) battery cable prior to beginning any work on the electrical system of the motorcycle.
12ga wire, with an inline fuse, required from battery Positive (+) terminal to relay. FIAMM install instructions recommend 10A fuse with single horn installation. I installed a 15A due to heavier wire used for installation. For twin horns, use 25A minimum. For durability, I'd also suggest the weatherproof mini blade-style rather than a glass tube or euro ceramic fuse. NAPA part number 784667 for the mini fuse holder.
Relay Terminals and wire routing:
30 - 12V direct from battery to relay. Use 14ga wire, nothing smaller.
87 - Change over contact direct to horn terminal. 16ga wire.
85 - Extend original wire from horn (Green w/Black tracer) to this terminal. 16ga wire.
86 - Extend original wire from horn (Brown w/White tracer) to this terminal. 16ga wire.
A new wire is required from horn’s second terminal to ground. Ample grounds available near coil. Dual horns will need
jumpers for 12V and ground wires. Professional-grade electrical wiring, like coffee, begins with good grounds.
Space should be available to mount the horn relay just aft of and adjacent to the existing relays. Hope this helps with your horn installation.
Monte