Help, 1966 Olds 98

[Briley.mitchell2004]

Hello, I own a 66’ Olds 98 with Comfortron ac, I only recently got the car, fully restored and everything, and last night I turn on the heat and notice the fan speed will only turn on in the low setting, I moved the slider to defrost, turned on the cold air, did anything I could and the high setting will not come on, I have a book for 66’ that’ll tell me how everything works, but I don’t even know where to start on taking apart the dash, I might not have to depending on what anyone says, this is the 1st time anything like this has happened since I got the car in September, thanks