I need some help my car is a 1984 300SD (126)

the AC does not work and the heater use to work.

what happen was one day it went out, and so i looked under the fuse box and changed fuse #11. and it worked again. a couple days after that it would turn itself on and off by its own (i press the defroster button). sometimes it would come on, then go away, then come back on in random intervals. when it doesn't work its not just the defroster button, but the whole heater system.

and it blew a couple more fuses. now it doesn't blow the fuse, but it does work either. could some one give me an advice on what i could look into? its started to get really cold here and i need the defroster to work in order to drive. thanks for your advices.

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198? 300D ???K - Future DD
1989 300TE 285K - traded in '12
1984 300SD 186K - Sold in 08
1978 300SD 330K - Died in 05