Heater won't blow hot

[68 ReNay]

i've got a 68 C-10, 350, auto, (non-AC)....

the heater blows luke warm at best and the blower would come on and off (loose prongs on heater switch), so...

i replaced the heater core, heater switch and all the gaskets (pulled off interior and exterior boxes). now all the speeds work great, it blows really hard too - but - still won't blow hot. i noticed that the heater blower speed resistor is falling apart (original cardboard type base). would that have anything to do with it. by it's title it doesn't seem like it - but i can't imagine what else the problem could be.

(my old 72 would sweat me out of the cab in winters. this 68 won't even defrost the windows)

any help is greatly appreciated!!!