You can find a heater control switch from Kanter (kanter.com) that will handle your speed control needs. I think it is about $30 plus the cost of shipping and as I recall it is rated for 20 amps. It works using resistance wire set into a ceramic holder (so heat can be absorbed/shed). Please don't try to convert 12v down to 6v. There are significant unknowns here and at the least you will have to "dispose" of a bunch of electrons in the form of heat. If you look at any of the Chinese made fan controllers, you'll see they all have a finned heat sink on the back. Some are rated for 7 amp and others for 10 amp even though they look to me like the same unit. That amperage rating and the heat sink is to prevent the equivalent of Mount Vesuvius from happening. The heat sink gets hot and the fins shed that unwanted amperage as heat. Your fan motor...we don't know the amperage and it may be as high as 10 amps or more (a friend had one last summer we measured at 12 amps spinning with the squirrel cage mounted on it)...might not work with a 4 amp voltage reducer as some folks are selling. If it draws 10 amps and you run that through a device designed for 4 amps, well, that heat has to go somewhere. Even if your fan only draws 5 amps (and I suspect it will draw this much at the very least), you want the device to be rated for at least 50% higher than that. Good luck.
