Where to mount a brake controller in a 2014 Toyota Sequoia?

First recommendation is to get a Tekonsha Prodigy controller; they can be mounted up to something like 30 degrees; don't need to be anywhere level. Then, while you're at it, order the Tekonsha pigtail adapter for Toyota (about $15) and the entire install is plug and play, no wires to splice.

Post your photo if you can and we can give you more info as to where to install.