What type of vehicle? If it is a TJ, I had that problem on mine. The selector switch that chooses what vents or defrost you want was the culprit. It has vacuum lines and a 3 wire connector on it. The electric connector acts as a "permissive" to allow the motor to energize. Mine burned up. I ended up making a pigtail for it. I've also seen on the internet that the resistor block sometimes gives out, but usually that just means you lose 1 speed first, then you may lose another speed, and then another, and then another.
