How do I make the heater fan blow harder in my 1975 cheavy pick up?

[paul_nagella]

When I turn it on bilevel you can hardly feel it

 

[kelly_f_1999]

most likely door is not working right or a hose is off or plugged

 

[KillerTaco]

check the voltage or is air is leaking from somewhere else

 

[jonesj82]

possible problems heater core hose is plugged flush with water hose or could be switch is bad or blower motor is going bad possible bad resistor switch more than likely blower motor is going out