check the diverter door in the air box (ac to heat) inside van pasenger side front under the dash. ford has problems with the air box 97-99 in the vans. i had to replace one in a 97 van. the part was 200 from the dealer. the replacement part is for 2000-up. this should have been a recall. i talked to the parts guy at the ford dealer he told me they sell so many of these they cant keep them in stock. when you move the control for ac and heat on the dash it moves the door in the air box.(little electric motor) if it is working you will hear the door slam into position both ways (door up-door down) it might be jammed (stuck half way) move the control to ac and back to heat a few times might un jam it. the door on mine jammed and then broke (hinge) your water pump should be fine (no leaks and not froze up) try what i put in this post and let me know what you find. i will help you as best i can.
Is the control for the diverter door manually or electrically or vacuum controlled, or what in my '97 E-350? The temperature control is the dial type, not the slider type. I don't hear any noise when changing temp from hot to cold. The rear heater is working fine, but it sure is cold on the driver's feet!
That means your blend door is either broken (common problem), or the gears are stuck (another common problem). If you look up under the dash board, on the passenger side, all the way to the left, look up high, you will see a white gear box and motor assembly. Tap on the box and if it is just the gears, the tapping will loosen them and you will get heat again. I've been doing this now for about 3 years.