How to get my Heat a/c fan to turn on high speed?

It will come on low & med but not high speed. I put a rallye dash in it over the winter and got to work the bugs out. It worked before so I am over looking something. Were would you look first?

On your heater box is a blower motor resistor that is actually the speed control. A bad or dragging blower motor usually causes them to go bad. Blower motors get run on high most of the time. A blower motor with drying bushings will have more drag on high than at low speed. That causes more heat than it can handle & burns it out. Bob

on my -70,i only have 2 speeds.rally ac panel.