Here is a cheap fix. My brother just bought a used 1 ton van for his plumbing company. It had an auxilliary heater in the back which he removed because it was in the way. Search wrecking yards and you can probably find such a heater at a very low cost. Mount it on the passenger side and run the hoses that originally went to the internal heater core to this unit. Next, go to an auto parts store and get a fan that mounts on the steering column. Point it at the windshield and you have a defroster. This was the heating a defrosting system for many cars in the 1930’s and 1940’s.
If this $50 solution isn’t acceptable, you’ll have to pay the big dollars to change the heater core. There were certain advantages including ease of repair to the way cars did things in the 1930’s and 1940’s. Changing out a heater core was no big deal. Now it can be a day’s work.