OER Parts offers a wide selection of Heater Parts for your 1977 GMC All Models. OER Parts offers 1977 GMC All Models Blower Motor, 1977 GMC All Models Heater Box, 1977 GMC All Models Heater Control Valves, 1977 GMC All Models Heater Cores. 1977 GMC All Models Hoses and Parts, 1977 GMC All Models Vent and Duct Parts.