2004 Chevy Blazer

I smell antifreeze through my vents. I had it checked diagnostically and "they could not find anything" and suggested a new radiator cap. That did not help. Now I have someone else telling me, it is the heating core (just by my description). Shouldn't they have found something when they hooked it up to check it? And I realize a heating core is labor intensive, how much am I looking at to pay for something like this? And how dangerous is it to drive until I get it fixed?