1998 Ford Escort brake lights stay on

I have a ford escort. The brake lights were staying on. I looked down around the pedal and found a little blue (looks like a little blue disc with a tip sticking out about the size of nickle) disc had fallen out and was broke. It seems this keeps the brake lights from staying on all the time. Do you know what it is called so I can tell the parts dept what I need and can I install it myself? It looks easy.