morin is correct about the small patch of cloth soaked with bleach.
While this method will help to clear the tube, I think a better fix is possible.
Iv'e noticed that quite a few owners have encountered a problem with algae and bacteria clogging the condensation drain lines in their a / c systems.
Swimming pool service technicians often come across the formation of green, blue and black algae in swimming pools.
To help combat and eradicate or to control the algae formation a liquid algaecide is added to the pool water in solution.
Once the algae is eliminated, and the pools filter has been able to remove the dead algae through filtration cycling.
A percentage of the algaecide remains in the water in suspension to help inhibit the future growth of algae.
For automobiles, several manufacturers currently offer aerosol sprays which remove unwanted odors and bacteria from the evaporative case and the a / c condensor.
The main problems with these sprays has been application. Sometimes the blower, resistor ballast, a / c amplifier, or inspection cover can be removed to apply the spray.
Therein lies another problem area, incomplete application and residue from the spray, in order for the spray to be 100% effective the interior of the duct system would need to be sprayed, short of removing the entire system and spraying every surface, then you have the problem with residue.
The spray contains compounds that allow it to cling to horizontal as well as vertical surfaces and while that may be desireable, albeit when the spray has done its job then the interior surface areas are left with a sticky coating that was designed to be rinsed off by the condensation that would be formed on a condensors fins as a result of condensation and surface evaporation.
More work needs to be done in this area to find a " simpler " solution.
In other words, short of removing and cleaning all of the surface areas in your a/c heater ducting system there is the possibility that this will be a reoccuring issue.