Do you have the heaqer box out of the car? What kind of car is it?

For a 124, at least...

If you are only replacing the heater core, you do not have to take the box out.

If you have the box out and are trying to open it, you must remove everything from the outside along with all the little clips. The box will then split down the middle but it my be stuck together a bit

__________________
2012 E350
2006 Callaway SC560