'91 300E - Heater core blues

[Rick Hall]

Need some help from the forum here. Started in to replace the heater core in my 1991 300 E (124.030). Have the dash stripped out, center brace out, left side off heater case, top plenum off, and right side completely off. I removed the bracket behind the heater case and rotated out of the way. I have the top of the heater core exposed and have taken screws out of the left fitting. I have one screw out of the right fitting. Unable to get out the second screw without taking off the top of the case, which I cannot get accomplished. I dont see any screws but I guess there has to be something holding it on.
QUESTIONS:
Do I have to take this top off to get the old heater core out and new one in?
What am I missing at this point?
I can send pictures if needed. I am so close but can't quite get this figured out, I'm stuck! So close but so far!!!

Any help appreciated!
Thanks
Rick Hall
Dallas