My subdivision suffers from frequent power outages, so I want to make sure I stay warm this season.

Parts that I have, 1 automobile electric radiatior fan, duct work box to mount the fan, a pair of Gator Clips, 100ft extension cord.

I mounted the fan with its housing inside of the duct work box, and cut 4ft off the male end of the extension cord, this I wired directly to the fan, then I plugged the cords own female into the male end, and at the other end of the 96ft piece of extension cord I put the pair of gator clips and clipped it to the 12V car battery.

The system works but it is all or nothing, and I would like to be able to vary the speed of the fan so that I might be able to adjust the airflow. What can I use to accomplish this, I have thought about a Dimmer Switch, or a 3 Speed Switch (low, med, hi) like on a fan.

Please can someone out there help me.

Neither, since it is a 12 volt DC circuit. Those controls are designed for 120vAC.

You will need a motor control that is designed for 12vDC, and/or something that will drop the voltage.