when you say drain, you asking about the drain for the EVAP coming from the airbox?
Id check you cab filter under the glove box and your air ducts. filter is probably rotting away or you have a mouse in a duct or something. Easy temporary fix is dryer sheets in the dash vents.
if the filter is rotting away some of it could still be in the airbox and realy the only true way of cleaning it out is to take it apart.