Brake Lights On Then Off

The brake lights on my 97 GMC Sierra are not working right. When you apply the brakes the lights come on when you first press the pedal but then they immediately go out.

I'm thinking brake pedal switch. Any other possibilities I should look at?

Jump the switch see what happens.

Is it a simple 2 wire switch? Of course it's up on top if the pedal where its not easy to get to.