The 30 amp converter (DC to DC...12v to 6v) will be able to handle the amperage (current) requirement of your heater motor and should stop the smoking. That is assuming the heater motor is in good nick and doesn't have some internal short or something.
However only if you have a 2 speed (or 3 speed) motor will a multi-position switch be able to make it run correctly at different speeds.
Hoping not to be boring, but a 2 speed or 3 speed motor is internally wired to be that way. A 3 speed DC motor for example can run at three different speeds because of field (or maybe also armature) windings that are different. These windings produce different magnetic effects inside the motor and the neat thing about them is they are designed so that all you need to do is to connect 12v to one of the wires and the ground wire to a good ground connection. When that is done, the motor will run at that same speed without getting overheated. This is how the best Heat/AC motors are made.
The alternative is to feed a simple DC motor less voltage. That will reduce the speed of it. But if you are at the same time converting 12v to 6v, please remember you have to find a way to "waste" 6v at higher amperage. Electricity isn't like flour...you can't just measure out a half of a cup to get 50% less. You have to send 6v off into "wasted voltage/amperage" land and that creates heat. So even if you have a really well-designed voltage reducer you will also have the amperes to discard.
A 30 amp DC to DC 12v to 6v converter will have 4 wires if it is designed correctly. If it doesn't have 4 wires, then I doubt it will be able to handle more than 5 amps. There will be 2 black wires and two colored wires. You will send 12v positive and 12v ground in using 2 wires and you will get 6v positive and 6v ground out from the other 2 wires. And you'll be able to power a motor that will use 15 or 20 amps comfortably...but the converter will get hot and it will have an aluminum heat sink.
For decades the best explanation I've come up with is this: please think of voltage as the size of your arm. Think of amperage as the strength of your arm. Mama Cass's arms were probably twice as large as Chuck Connor's. Which one would you rather arm wrestle?
