rust stain from a/c unit driver side

Maybe I'm confused. Which "problem" are you wishing to solve? There are several ways to solve water run off, from placing small plastic strips held on by dicor to drain hoses that drain off the side when in use. If it's the discoloration that's bugging you, some rubbing compound, (lightly) on the outside will get rid of the discoloration. You might want to start with Dawn liquid and move up from there until it's gone. Vinegar and baking soda works well too, including your foggy headlamps. Use caution though as mixing the two will cause it to foam like a shook up soda can. There's no danger other than making another mess you'll have to clean up.....Dennis