I had a used car dealer call me for an Evap system problem in a Ford F150 4X4. When I got there I attached my Ford Factory scan tool and checked the codes set. The code was for the vent valve circuit in the evap system. I went to the back of the vehicle by the rear end where the vent valve was located and commanded it on with the scan tool. It did not react to the command from the computer. I then pulled it out of the vehicle and bench tested it. It tested ok so I reinstalled it and started checking the wiring for that circuit. There was no power on the wire to the vent valve that should have had power. I then checked the wiring at the computer and the wires comming out of the computer had power and ground when commanded to. This told me the computer was working fine and the problem was with the wiring. There were three connectors in line between the computer and the vent valve. I went to the first connector in the left front kick pannel and I had power on both sides of the plug so I knew that connection was ok. Went to the next plug and did not find any power on the red wire so I went back into the cab and rechecked the first plug and it still had power. I then removed the plastic trim piece covering the rocker pannel inside the vehicle. When I did that I exposed the wiring trought that was submerged under water. I dug the red wire out to find a buldge in the insulation for the wire. I cut the insulation and found a green belly of corrosion and juice on one side of it but not the other. I repaired that wire and three others with the same problem and advised the customer to find the water leak to prevent this from happening agian. We then went to the scan tool and activated the vent valve and it worked. Vehicle fixed and customer happy.