I would imagine that you are asking about an older vehicle heater, since there is a big difference between a heater that uses a blower motor resistor, than a system that uses a computer controlled automatic heater system. In an older one with a blower resistor, most of them are wired so that the current must travel through the resistor on the low fan settings. On the high fan setting, the current will flow through a high speed fan relay, so that the blower motor receives full system voltage.

Now if the system is an automatic climate control system, the HVAC computer will most times have control over the motor. Most of these systems with control the blower speed by pulsing the current to the motor.

If you want to know how a specific system works, print out a wiring diagram of the system and study the diagram.