Just hope and pray its in the front screen and not futher down the food chain.
under the dash in the vents , blocked vents and rust. yes just what you want.
if it has all the above ,, park it ,,sell it ,, or raid what you can of it and dump the shell in the bin.
All the seals perish and welded seams give way over time and the its near impossble with out Major good old fashion hard workto get it fixed proper.
front screen removed and replace if ,, it breaks.
all or lower half of dash , vents , hoses , cables , carpets, pedals if need be.,, wiring ,
depending on whats in the way under engine bay ,
always cover area with board or big old wey sheet ,
then you can start cutting welding
priming filling painting
put it all back to the way it was ,
and it still leaks.
check all options before the above ,, it can be done but depends on how much you love your car.
its probly just a seal some wear.