No heat in my trooper. whats the deal?

i drive a 1989 isuzu trooper II to and from work and this winter i am not going to put the card board in the grill unless its my last resort. should i replace the heater core? i already replaced the theromastat and it did not help. it blows semi warm but not enough to melt the snow off my window and not enough to keep it defrosted half the time. plus i dont want to ride around in my carhartts anymore. lol any help/info would be nice. thanks