In my experience usually no codes.
The tailgate switch is basically a resistor and a contact switch.
The BCM monitors the voltage on that wire and when it sees the voltage drop to a certain level it knows that is the signal to pop the tailgate.
Its called resistive multiplexing.
If the resistance of the switch is off (usually from wear or corrosion) the voltage is off and the BCM doesn't know to pop the tail gate.
Usually no codes unless the switch shorts out.

Sorry don't know any cheap online sources.