The original switch should look like the ones DADS50 posted they come in a few different styles some say fan, some say heat, air etc, and there are subtle color differences. They are manufactured by a company called Cole-Hersee and they are still in business. You can still find them NOS on places like eBay $$. The other styles mentioned do not appear in any of the factory literature I have while this style does.
Here is a picture of a switch on a 54 this is the correct location for the heater switch the dash is drilled here for two switches to go side by side one for the heater another for options like vacuum pump, fog lites, etc.
http://community.webshots.com/photo/2503692070102454676pgoIoi
