I installed defrosters in my boat just this past spring just for this reason. It is a heater core connected to my closed cooling just like a heater core in a car. I purchased the heater core in its own box with a three speed blower. It then has two blower hose tubes going up to two vents installed in the dash under the front windows. It works like a charm. If you need pictures i can get them for you as well as a website that sells all of the products to do such a thing.