Prepare for a battle. Once mold gets started in the A/C it can be very
tenacious. For more info, try
http://www.autoacsystems.com/_faqs/detail/ac-faq39.html and the links from
there.

A popular DIY approach involves flushing the evaporator with alcohol of some
sort. Rubbing alcohol is satisfactory. The basic procedure is:
*Locate the drain tube under the car; it is usually near the firewall around
the passenger's feet. It will be a plastic tube about 1/4 inch diameter that
disappears into the passenger compartment. Whan the A/C has been operating
the end of the tube should be wet and probably dripping cold water.
*Use tape or similar to attach some squirter (large syringe, turkey baster,
etc.) filled with alcohol to the bottom of the tube, then force a few (about
2 - 6) ounces of alcohol through the tube to flush debris out, then remove
the injector device to let the alcohol drain out.
*Locate the recirculate intake of the air conditioner. It is normally under
the dash near the passenger's knees.
*Put at least 8 oz of alcohol in a spray bottle.
*Set the A/C to maximum fan and recirculate (usually called "max") . Set the
temperature full cold. Spray the alcohol into the intake - the more the
better.

If all goes well a lot of the mold will be flushed out and much of the rest
will be killed by the alcohol. Be sure to let the car air out for a while
afterward :-)

Be sure to keep up with the cabin air filter changes. A common misconception
is that it is there primarily for the occupants, but it is primarily for the
health of the evaporator.

Mike