Find out where the drain line goes. Use a wet/dry vac to vacuum the
drain from outside. This way all the water and slime and crap is
pulled out. Then treat the drain pan with a little Clorox. I usually
spray coil cleaner on the coil, Greased Lightning would work reasonably
well. Mix the cleaner with water per the directions. Don't use
anything corrosive, it will eat the coil.

Stretch