I installed mine attached and above to the center bracket for the rudder peddles and the cable comes right to my panel.
Another possible and good location is right behind the subpanel where the brake hoses are suppose to turn downward to the tunnel. The only negative part about this location is that in most cases there is good bit of wiring for crossing this section. Mine has worked flawlessly so far.
Starting on page 8 of the manual linked below, is the location that our hose kit has the parking brake mounted. It's definitely not the only way to do it, but it does give a visual of where our beta testers placed the valve and where we have customers place the valve when they want to utilize our pre made brake packages.