Good morning, about 2 months ago I had to replace the heater core in my 2006 Chevy Silverado. There is a top wire harness on thetop of dashboard I cannot figure out where it goes. Any ideas?

This is the harness. No label for it.

 

There are often unused plugs found as harnesses are set up for use in various models with several options. Likely goes to an option your vehicle does not have.

 

As stated, these are generic harness's that cover everything option available, your truck probably didn't come with the unattached option you have. You can survey the area around the plug whatever is there that needs plugged in with be within it's reach.