What's the possibility that my Engine Control Computer is bad in my Ford F-150?

I was deployed and just got back home after 3 years to my truck, and now when I try to turn the key to ON it blows my PCM Fuse, I hired a mechanic and the wire harness is good no shorts anywhere. It all seems to boil down to that. Possibly likely? What would cause it? Sitting in the sun for the 3 years?

Truck is 35" lifted, its big had been baking in sun.

1999 Ford F-150 Lariat 4.6l V8

Slight possibility, but id look for a short caused by corrosion, but id look for rodent damage as they love eating wires and shorting them. But id probably get the wiring diagram, start unplugging stuff on the circuit and keep trying fuses till it won't blow. As pcms normally won't draw that much cuurent to blow that fuse unless its very hot to touch