I have a 2001 chevy hd2500 duramax 4x4. I have

I have a 2001 chevy hd2500 duramax 4x4. I have the cargo light and fog light switch (all in one). The cargo light works fine! I cannot get the fog light switch to work (the light will not come on). I've replaced this switch, checked the relay (the relay tested fine) under the hood (in the fuse box), replaced the 15A fuse (same location), checked to make sure I had power at the fuse and relay with a test light. Still, no light on the fog light switch itself when I press the fog light button. I tried it with the truck running with no high beams turned on. As I understand it fog lights will under work with running lights and low beams. Is there another relay/fuse that could be broken? Any help would be appreciated as frustration is strating to set in!