What can i do if my radiator is broken? Its not the big one, its the small one for the heater....Lesabre 1967?

I have the lesabre 1967 and i was driving when sudently somdoby drove besidem e and told me the car was leacking water......i steped out and i looked..... and yes...its the small radiator that goes to the inside of car.....for the heater...now the inside of the car got wet...so i thought it had to be that.........how can i fix that?