I found this diagram for the brake fail light switch for a ford on a ford forum
Apparently the circuit is wired from the "run" position on the ignition switch, thru the indicator lamp, then to the switch on the "combination valve" or proportioning" valve which grounds when there is a pressure difference between front and reat brake circuits.
There is also a connection to the starter relay which causes the light to light when the ign switch is in the "start" position.