Searching on that "voltage reducer" number what I find is a 50 watt 4Ω resistor.
It's not a fancy part, just a resistor.
Check it with a multi meter and see if it has about 4Ω resistance. If it has lots and lots of resistance, or an open circuit, it may be burned out. If it checks out at 4Ω then it works, and perhaps it's not the right value for what you're trying to do.
A 12V motor will run fine on 6V, but it'll run slow, as if you have the fan switch set to "low". I presently have a 12V heater blower motor in my 6V truck, works just fine, but will work better when I upgrade the rest of the truck to 12V.
A 6V motor will also run on 12V, but will run fast and won't last long.
Take the motor out if necessary and get a look at it, may say what voltage it is on the side, or a part number and you can look up what voltage.
If it is 6V motor then you need a resistance large enough to drop the voltage measured at the motor to 6V.
Or just swap the motor out for a new 12V motor, best option in the long run.https://www.stovebolt.com/ubbthread...6055/Easy/cheap_heater_blower_motor.html
In any case the voltage at the fan motor should be 6 or 12V to suit the 6 or 12V motor, what do you measure?