The brake pedal switch is probably bad. There is a physical switch on the brake pedal (that triggers the brake lights). They get a lot of wear, luckily it is a very simple DIY and only costs about $20. I replaced my clutch switch (basically the same switch, location, etc) to solve my cruise control not triggering, takes about 5 minutes.
Link:
Pelican Technical Article: Boxster Brake Pedal Switch Replacement / Clutch Pedal Switch Replacement - 986 / 987