Sounds like your resistor on heater box under dash is bad or disconnected--3 wire connector--also check heater fuse as high speed on blower is on a inline fuse.
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56 Chevy Bel-Air 2dr. HT (purchased new)
71 Chevy Cheyenne SWB PU (502HO)
65 GMC short bed step--work in progress and my gofer
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