Visually, if the motor pretty much covers the entire hole on the firewall, as if it belongs there, chances are its the original motor. What you need is a switch like the one I pictured here:

http://devestechnet.com/Images/Projects/12VUpgrade/12VUpgrade0006lg.jpg

then a reducer like this one:

http://www.speedwaymotors.com/Voltage-Reducer-for-Fans-and-Motors,2375.html

put the reducer before the switch so it encompasses the entire circuit or you will have the same problem you have now.

If that works for you great. If not, that instruction I posted above will help you change to a 12v motor. Even if the heater is different, the motors are the same.

Deve

1950 Chevy 3100 Deluxe Cab
1950 Chevy 3100 Standard Cab
In the Stovebolt Gallery
The Think Tank
More info and tips at Deve's Technet