What is this object on the tail of an F-16?
Source: primeportal.net; cropped


That's the cooling-air outlet that is aft of the cooling inlet. In between is a power amplifier for the radar-warning system.


  
  Source: amazon.co.uk (one of the product images); cropped
  
  118: Cooling-air ram air intake
  124: Radar-warning system power amplifier


Related: What is the device at the tail of the F-16 Block 60?