So you went to the dealer so they could "fix" the heater core but it wasn't working at all when you got it back... did you pay these guys for this too?

Check to see if the hoses are attached right, including the vaccume lines. Flush the core and hoses. Sounds like something is plugging it up, the system itself is very simple.. just tubes of water to transfer heat into the air which blows by it.

This sounds like a vaccum line issue.