There are no filters.
You probably need to take apart the dash and remove the heater box assembly and disassemble it and clean out the heater core. Leaves and crap comes in through the cowl vents and get down on top of the heater core and when the blower tries to blow hot air through the core, it is restriced and you won't get much air flow through it.
Some people drill a large hole up above the heater core in the side of the heater box and use a vacuum to suck out the leaves and crap from off the top of the heater core. That works good for removing the stuff from on top of the heater core, but there is likely a bunch of crap in the fins of the core and the only way to get it out is to take everything apart, so you can vacuum, blow, or wash out the heater core fins.