If you can test it, remove the pollen filter and leave it out. Start up the fan and does air blow from that filter? It doesn't tell you exactly where the issue is but if nothing comes out there, blocked intakes. If something comes out, then blockage from filter box to vents.
I'm trying to help out a friend. Their heater motor stopped working so I fitted a new motor and it works no problem, but when I checked at the air vents there is no air coming through but I can hear the fan spinning fine. I have checked the in cab "pollen" air filter and it isn't blocked. On closer inspection to the new motor I can see the fan blades are curved in the opposite direction to the one taken off and the direction of the spin is opposite also. The motor factors I used said i would have to try main dealers for the right one. Has anybody had a similar problem and can shed light on it? Thanks Jon