Try a light puff of air from the a compressor or air tank thru the heater core inlet. Sometimes they get gummed up with crud and/or rust.
Just a light puff, you could wreck the heater core.
Had to do that on my wife's '98 GMC Jimmy.
It solved the problem.
It will cook you now in the dead of winter, (-20 below or more.)
Just my .02
CCDually