Please, can anybody give me advice on the following:

I own a 309 (1989), and my heaterfan recently started to make funny noises.
It turned out that there was no lubrication what so ever on the axel from
the little motor. No problem so far... that I took care of.

But the main problem remains... Although I tested the heater motor directly
on the battery, where it worked great, I still do not have enough warmth
coming up the inside of the windscreen. I have already checked the valves
that lead the air stream to the windshield, or to the side windows, or to
the floor. They also work fine.

But, I just don't have enough air flow... :-((((

Please, think with me:

If a fan can't suck IN enough air, than it can't blow enough air OUT, isn't
it?
My theorie:
Could it be, that the heater radiator is clogged up with i.e. dust, leaves
or anything like that, where is tit, and how do I clean it??

Anybody an idea?

Cor