Where is the heater blower motor in a 1971 cutlass the blower only works on the first two speeds not the highest

Blower motor resistor most likely.

The blower motor resistor is the component directly responsible for controlling the blower motor fan speed. If the resistor shorts or fails, it may cause the blower motor to remain stuck on one fan speed.

Video of a 2011-13

https://youtu.be/K4GEU4GKjD4

A heater blower motor resistor is an electronic device with three or more progressive resistors that allow the heater blower motor to operate at slower speeds. The highest speed of the heater blower motor is not controlled by the resistor, so when the resistor fails, the fan will often work on the highest speed setting, but not the lower speeds.
To resistor is generally located under the dashboard and under the hood, near the blower motor . The resistor's electrical connector is disconnected, the resistor is removed and the new resistor is installed. On some vehicles it may be easily accessible, and on others it may take partial dashboard removal to replace the resistor.
Look under the hood, generally the wiring for the Blower resistor is attached to the wiring to the blower motor itself
It's inboard & below the motor, near the accumulator. You can see it standing beside the passenger fender. it's the off-white connector behind the L heater hose above the vacuum reservoir & the white WARNING sticker

The resistor is most likely bad but should still work on the highest setting.Would check pigtails to the resistor and to the blower motor for burnt or discolored wiring.You can try tapping the blower motor and wiggling the wiring harness too see if it will work on the highest setting. It is possible that the blower motor is bad as well typically when you lose the speed selection it is the resistor but you should always have high speed.

hte blower resistor is shot, near the blower motor, have the blower motor checked my guess is that it requiring too much power to run, hence the blown fuse

this sounds like you need a new blower motor, check fuses first if not blown replace blower motor

The resistors are in circuit only on the lower fan speed settings.
On the highest speed setting, power is switched directly to the fan motor.
Check fan power supply fuses,
Check power is available to the fan when high speed selected.
If all above OK, apply power directly to the motor to verify fan motor runs- if it doesn't the motor may need replacement/repair.

heater blower motor risister most likely cause. should be under dash near heater motor.

Usually when the blower loses speeds over time the problem is the blower motor sensor.luckily these are relatively cheap.

this is caused by the blower motor speed control resistor being defective, this part is located near the blower motor.