Heater fan speed won't go on high

[Orion88]

My 71 Chevy has a very weak fan speed. My first assumption was a mouse nest somewhere in the ductwork or blower motor. Took all that apart and to my surprise it was mostly clean. I then took out the heater control panel and messed around with it. When I go to turn my fan on, nothing happens until I get to about 3/4 full speed. Then it finally starts blowing...barely. By the time I get to high it only blows like it's on a low-medium speed setting. My guess is the cable needs to be adjusted, but I couldn't find anywhere under the dash where you can adjust the fan speed cable. I don't think it's the relay because it would not work at all on certain settings. Instead it acts like the cable isn't engaging the fan until 3/4 of the way, so it simply cant blow on high speed because the cable is out of whack. Does anyone know how I can adjust the cable, or if it might be something else? As brutal as this winter has been in the midwest it sure would've been nice to have a fully functioning heater. Better late than never I guess.