The "backup" fuse is not going to be any use to you as it is fed direct from the battery via the main fuse. It's there to power the immobiliser and keep the clock's time when the bike isn't running. In other words, it is a permanent live - not ignition switched.

You're best bet would be to wire up a relay to take a trigger feed from the supply to the brake light switch or tail light under the seat, then take the load for the grips direct from the battery via the relay and an in-line 5A fuse.

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