1977, 300D, 123 chassie

1977 300 d 123 chassie Would like some instructions on troubleshooting the heater and air cond. system. The blower comes on only for the defroster.Is there a recommended change over for the 1977 air cond. unit to be able to purchase the freon. I am not good using facebook but I am trying.
Thanks, Karl Stier

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