Where to mount a brake controler in an 80 series

Hullo Trevor
Mine is mounted between the steering column and the facia approx level with the heater controls.
I think the suggestion that is must be horizontal or as level as possible, is a bit of a furphy, as the people who fitted mine put in about 2 a week (in a range different vehicles) and know their stuff.
Mine works fine at about 60 degrees
The main thing is to have it handy so that you can readily use the hand slider control for activating the trailer brakes independently from the foot brake and that it is accessible for the passenger as well, as there are times when it is an advantage for them to operate it for you.
Cheers
Andrew

The limits for the mounting angles on the earlier Prodigys are:

Side to side they must be level.
The fore and aft centre line must be parallel with that of the vehicle.
The fore and aft angle must be between horizon and 70 degrees up at the rear.