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I posted the exact same question a week or two ago. I go very few replies but what I have thought about doing is taking out the cigarette lighter and mounting the controller in that little oval shaped area. It's out of the way and it's easy to get to in an emergency.
I guess there is no "normal" place for one. Just depends on how comfortable a reach when needing to manually add a bit more brake. My trailers weight differs quite a bit from day to day. I like a light trailer brake most of the time but I could not reach the controller comfortably in a panic situation if it was under the dash. It fits nicely in this compartment and if you drill a small hole in the back you will not see any wiring.