The main wiring harness in the trunk of a 2005 Chevy Cobalt is typically located
on the driver's side, often running along the inner side of the wheel well and/or under the carpeting near the taillight assembly. It's bundled together and might be partially hidden by other components. You'll likely see a number of connectors and wires grouped together in this area.
To be certain, you might need to consult a repair manual specific to your 2005 Chevy Cobalt. A Haynes or Chilton manual will have diagrams showing the location of major components, including the wiring harness.
