the a/c fan blows but not alot coming out of vents

i am trying to figure out how to get to the heater core/evaporator to try to see if it is clogged but am having a hard time finding any info on this subject.i just dont wont to tear more apart then i need to.also the fan works on all speeds but it really does come out the vents unless you have it on high any help or info would greatly appreciated.