To activate the system with the brakes, instead of going from the brake lights,you could most likely go straight from the brake light switch. If the make/model of your vehicle had the option of cruise control than your brake light switch likely already has a terminal that is activated (or deactivated) when the brakes are applied, to shut off the cruise control.
I uses this when I installed a lock up torque converter in my El Camino, as soon as I applied the brakes, the "cruise control" terminal of the brake light switch would automatically disable the cruise, but I use it to unlock the converter.
If your car does have the option for cruise, this would be a decent way to have a factory connection/ switch.
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