What the hell happened to the OP?
To double check if its your heater core you can also do a radiator/cooling system pressure test. You can rent the testers for free at local autoparts place.
Just attach the pump to your radiator cap location and pump it up to around 15 psi and wait. If it drops in pressure and/or the mysteries fluid starts pooling up in your floor boards, ITS THE HEATER CORE.
You can also do a similar test by disconnecting the heater core lines. Blocking one of the heater core ports and putting pressure through the other port to see if it holds pressure. This is what the dealership should have done........its in their service manual.
I think some places carry a heater core tester. Your setup should look similar to this. Notice the quick disconnect on the one end. NOTE you shouldn't apply more than 15 psi (I wouldn't even go that far maybe 10) otherwise your core will blow. You need to use something with a gauge to check.