Defroster Ducts and Vents

On my '64 The stuff falls in and gets blown through the heater/vent system.
There are two plastic oval plugs on the firewall. These openings are for clean out access. Size a somewhat ridged extention hose to a shop vac and clean out by vaccuming. Also there are the vent drainage holes [slots] between the body and fender -open the doors to see-and 'access holes' on the engine side. Use a plastic coated electrical wire to clear. Alternating with the vaccum set-up helps. I found out that all the years' of crap had rusted out the vent/heater "stacks" right and left. That became puddles on the floor after every rain= Another story.
Back to the debris problem: Take your heater out, open it up and clean. While it is a "fun job" involving the heater core hoses, as well as the cable and electrical controls, Having a good clean running system is worth the work.
P.S. Look under your doors and (on two doors the rear side windows-under body) for water drains. If they do not drain the water that runs down the side windows alot of body damage can occur.