AC Box Rebuild-Missing Flapper Door

340sFastback said

Blower fan resistor is on the top by the heater core on my '69 HVAC box.

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Yeah, the resistor is intact. Actually, this blower switch plugs into the blower fan resistor. So in know it has something to do with controlling the fan. I am assuming it turns it on high when either the defrost or max AC button is pushed.