Most likely the air leak is originating from the small Unloader Valve, where the 1/4" copper line connects to the pressure switch in your photo. It probably seems like the air is leaking from where the power cord installs into the switch but most likely it's just deflecting from the Unloader Valve.
Does the leak occur when the unit is running? Or, only when the unit shuts off?
If it leaks when the unit shuts off, most likely your tank Check Valve is defective.
Click on the below link find out how to test the Check Valve; there is a video and a document which will tell you how to test to confirm the Check Valve is bad or not:
The Check Valve is the main one-way valve in the tank inlet which holds the air in the tank after the unit shuts off; if the Check Valve is stuck open after the unit shuts off, air will constantly back-feed through the main Discharge Tube and through the 1/4" copper Unloader Line. It usually leaks out of the small weep hole on the side of the Unloader Valve. This is indicative of a bad Check Valve.
