I have Hex EzCan and Denali "Brake Light" connected to it. I've chosen 10% of intensity to run as running light and 100% to actuate as brake light. That is the option if you don't want to touch bike's harness and splice into it.
The other option is to purchase Denali "Brake Light" only. It exists in few sizes and mounting options. The light itself has one negative and two positive wires. One positive is for splicing into the positive of rear running light and it's low intensity and the other positive wire is for splicing into the positive of brake light for full intensity for braking if you wish. I think that may be a good starting point for what you want.