The following is deduced from looking at the Interstate wiring schematics. I don't own and Interstate, and thus haven't actually seen any of this, so caveat emptor:
Power from the accessory circuit in the ignition switch flows via a yellow/black wire to a single pin connector somewhere under the tank then via another yellow/black wire to the audio diode behind the left side cover. This circuit doesn't appear to be fused, other than by the 30A main fuse, so please consider fusing any wire you splice into the yellow/black wire from the accessory position circuit. Also, consider the size of the yellow/black wire when determining what you intend to power from this wire and what fuse size to use; currently it only needs to carry the power required to trigger the main audio relay, so it likely isn't very large.
See the wiring diagrams provided by Rattlebars. The relevant diagram is
here.