Well done for noticing the leaves - they have killed many an escort by building up in that area and staying wet.
As already mentioned, get the air compresser onto it. Blow into it in as many directions as possible until the leaves etc stop coming out.
Taking the heater out - budget about 2hrs for that mongrel job but as mentioned it will tell you if there is any rust there and also a good way of checking out the wiring situation behind the dash while you're there.
I pulled my heater out last weekend to and found just a tiny tiny bit of surface rust in the join between dash panel and firewall behind the heater - bit of a scape and a burst of primer for peace of mind sorted that out.