What causes AC condensate to leak on the passenger floor and how do you fix it?

What causes AC condensate to leak on the passenger floor and how do you fix it?
The usual cause is a condensate drain that is plugged. You should be able to crawl under the car and poke something up through the drain to clear it out.Another possible problem is the drain pan is cracked or broken allowing the condensate to drip. This would require partial disassembly of the dash to replace.