I have owned 6 different 65-66 mustangs over the last 28 years and back in the day when I was in highschool (80-83) I hated the defroster on my 66.
I used to carry a roll of paper towels in the car with me and when it was cold and damp I would wipe the moisture off of the inside of the window
I did not have a leaky heatercore or hoses, it was just humid,cool, Louisiana winters.
The previous poster hit the nail on the head.... new cars have dehumidified defrost air.... old mustangs don't.
I currently own 2 65 stangs a coupe and a fastback, but they are only fun good weather drivers so the defrost does not matter.
You can get a modern air/heater/defrost setup made for classic stangs that will work like a newer car, but they run around $1k for a kit.
Good luck