Blower motor has one speed: "On"

[kcackler]

This isn't really a problem, but rather just a curiosity thing. The fan speed on my Air conditioning/heater controls does absolutely nothing. The fan is always running so long as the truck is on (Which I understand is normal operation), but I cannot turn the fan speed up or down.

Just curious if this could be something simple. I plan on adding A/C back into the truck this summer (All the parts are there..just got to make them work) and would like to get this fan thing fixed as well.