Later trads use a plunger switch on the brake pedal, mounted on a bracket so that the brake pedal movement closes the contacts in the switch.
Vw use the same method, and doubtless many other manufacturers do the same.
In my early days of Roadster ownership, I fitted a relay, as noted above. Then later on I happened to have a microswitch from Maplins, so made a simple bracket and set it up like the plunger switch.
