How do you unclog the evaporator drain?

Go underneath where the drain tube comes out and blow air up the hose. It may clog again, often there is a build up of sludge in the evaporator case that plugs the drain.

What I do is, use a length of rubber hose such as a neoprene fuel hose that fits tight over the drain tube, then adapt the other end to my shop vac and suck out the debris.