I recently had to cannibalize two BMW 2002 heater boxes to make one good one.
I noticed that both boxes had an amazing amount of foreign stuff (leaves, twigs, acorns, etc.) in them. These things were clogging up the core. It would not be possible for such large items to get past the 'Squirrel cage' type blowers in the newer heaters.
After reassembling the heater box, I fashioned a cone out of wire mesh. (screening is too fine as it will impede air flow and chicken wire is too large) and attached it to the outside of the fan opening with plastic wire ties. That should keep the large stuff out of the heater.
Author: John Stern
Bavarian Bylines, July 1998
Pine Tree Chapter, BMW CCA