why wont my brake lights come on?

I would check / replace the brake light switch - you will find ti on the brake pedal mounting bracket (pedal box).

Yes, what Paul said. I had a 78 Ford pick up, and it was the switch, under the brake pedal

I agree with both answers above... but would also add that depending on if your car has been parked in a garage or in the driveway regularly you could have some corrosion on the metal plates that the bulbs twist into. Especially in the 3rd brake like. (It isn't likely that ALL of your brake lights will not work because of the corrosion which is why checking the brake light switch would be first...checking for corrosion 2nd.)