no air flow

i think i found the problen i took fan motor off and reached in the houseing its all clear but on the firewall i feel something like a rad but its caked with dust and dirt could that be how air gets in to mt defrost and vents, only thin is hoe do i clean it up

take a regular screwdriver and gently scrape the buildup then take carb cleaner and an old toothbrush spray the crap ouy of it wipe dry the best you can and your air flow will be damn near as good as new the smell from the carb cleaner in your cab is normal and will be short lived it is a tight fit but it is definatly worth the effort