What causes brake lights and turn signals on 1988 Chevy Silverado to not work Front turn signals work and tail lights work?

I'm not sure why the front turn signals would work but not the back, however I had a similar problem and had to replace the two relay fuses on the firewall just behind the fuse box. These will look differant than the regular fuses. They are aprox. one inch in diamiter squared and one is one in. long, the other is one and onehalf in. long. Good luck!