heating system question
 

My 81 300 SD has developed a glitch. The heater fan works fine on the heat and AC settings, but no fan when I switch to defrost. I'm guessing that there is a separate switch, since the defrost fan used to switch to high whenever I turned the defrost on, but I don't have a clue where to start looking. Any help would be great.

Thanks

Bob Hofferber
Vancouver WA
81 300 SD