[Classic Mustang] heater core?

I have a 66 coupe with at leat 200,000 and the beginning of the week i noticed a small puddle (way less than a shot glass full) of radiator fluid on my passenger side mat and i was feelin all around the heater and it was dry?! Today while i was letting it warm up before i left work on my way home (15 minute drive on freeway at about 70-80) i check radiator and it was low becaause i could feel the core? inside, however could it be low becuase i usually dont let it warm up going to work and i go down the free way at about 60-80 (tops out at 80).

thoughts,suggestions thanks in advance

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