How to Unclog Vent Drain?

[ColdCase]

Any tips to unclogging the heater/ac/vent drain(s) on a 2004. I blew some compressed air up what I thought was the drain tube about 6 months ago but that didn't seem to help much. I'm getting more and more water sloshing around and an occasional wet passenger foot I suspect a vermin made a nest in there somewhere that needs to be cleaned out, but I dunno.

Seems much worse after a heavy rain.....