You have identified the problem when you state that it only
works in the high position. Your switch has variable speeds, it
contains resistors that control the speed of the fan, for both
AC/Heat. These resistors are burned out/or the connector to the
resistors has become unplugged. The resistors are located in
different locations so please get your auto manual to locate there
position, and verify there operation.
Most cars have a simple blower motor resistor that provide the low-medium speeds. If it burns out the fan defaults to high speed - the most safe 'default'.
Walk into any auto parts and tell them your car only blows high and they'll immediately know what you need.
In most Ford vehicles this is a 10 minute, 2 screws procedure that most anyone can handle.