On a 1998 Silverado extended cab the brake light on the cab works but neither rear brake lights works what could cause this?

It could be something as simple as a fuse, or bulbs, or it could be that something has ethier shorted out you wireing, or something has cut it, you might want to check with you local mechanic, or Chevy dealier.

Answer

relay fuse under hood was culprit still only have right brake light