My A/c drain has been plugging up pretty regular since changing out the heater core last winter. Figured they left trash in the pan. It is plugged again but I can't get it open-even tried a shop vac with a smalll hose on the drain hole. Is there a way to blow the drain out from the other end? I guess it could be stopped up at the pan where the heater core and the ?condensor sit. Thanks for anyones help.

You might try shooting air into the drain to try to loosen junk then vacuum it.

I suggest using the vacuum first. Compressed air will just push all the junk inside further then it already is.