'71 Heater fan fuse blows instantly when key is turned on
 

Having an electrical problem with my fan blower circut. I've been trying to get the fan working to blow the excess fumes out of my car, but literally the second I turn the key to "on" the fuse for the fan blows, even if it's switched off. All of my electrical connections/grounds are clean and tight. I know the fan works because I jumped it from the battery directly and it turns in and moves a considerable amount of air. Could it be a bad switch?