Two possibilities: 1) it's the blower motor, located in the dash, most likely behind glove box. (Google where it's located). 2) It's the AC compressor, if you happen to be running the defroster and not just the heat. (The defroster requires the AC compressor to run as well, and the squealing is coming from that). Just verify the sound occurs with heat only and no defroster running. If it is indeed the blower motor, it may have something caught in the "birdcage" (the fan). Once you expose it, usually by dropping the glove box door, it's fairly easy to access.
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