I just open the taxi door and say "Soi Suan Pluu." If they give a blank look or worse, I just close the door and go on to the next taxi.
Just saying "Soi Suan Pluu" seems to work - they figure you're a foreigner and therefore, you must need to go to immigraton.
There immigration building is next to a police station, if that helps any.
There is a photogaphy shop on the same side of the street (exit the immigration office and turn left, past the police station maybe 50 to 100 meters) that can make passport photos and photocopies in a few minutes, for a very reasonable price.