I was under the truck running wires and saw what looks like a heater core or cooler right in front of fuel tank. What is it any one know. Thanks

Fuel Cooler

So why cool the fuel and when does it run. When the air conditioner is on or does it do its own thing?

Rescue7, the fuel system continuously returns fuel during engine operation. The "pump heat" added by compression to 20,000 psi in the common rail injection system and fuel bypassed in the injectors themselves add heat to the returning fuel. Fuel drawn from the tank is pulled through the Fuel Injection Control Modules (FICM) on each valve cover to help cool the electronics packages.