http://www.lmctruck.com/icatalog/cbe/full.aspx?Page=34
Part #19...heater resistor.
Plugs into the back of your inside plenum so its in the air stream to cool it. It provides resistance to the motor so it blows slow, medium or fast.
Pretty common on early Chevy cars too, my 62 Impala had one. $10.
Run it in the air stream....Bench test it with 12 volts and ye'll light one cigarette and ye'll smoke it (the resister). Ask me how I know.