Year, make and model of your vehicle?
On your vehicle the vent solenoid is located near the frame rail where the cab and the bed meet. They constanly get road grime and water on them. 9 times out of 10 a new vent solenoid will fix the problem. The 1 out of 10 is typically a wiring repair at the connector from somebody testing it at an earlier date.
You would need access to a class 2 scan tool and smoke machine to properly test the solenoid. It is shown in the picture below (#2).