Heater Blower problems

I have a 72 plymouth 340 duster my heater blower and dash lights will not go on .. I found a plug in under the dash thats hot but there is nothing pluged into it..Its a Two prong plug in with four wires coming out the back.. I put a fuse in it just to see what happens and when I did that my blower motor came on, but it ran constantly and the heater switch would not control the blower at all and my dash lights also cam on.. Now I just need to figure out exactly what plugs into that two prong plug anyone have any Idea?