Every now an then I try to turn the fan speed up for heat, the fan doesn’t always work, not sure if it’s a relay or contrllor?
Most likely the fan itself. The brushes wear at a slight angle and after a period of time, they get cocked in the brush guide and stick. When this happens, a tap on the bottom of the fan motor, located just outboard of the glove box under the dash and easy to see, will make the motor run again.
These motors are assembled in a way that makes access to the brushes almost imposible. It can be done but it is very time consuming. If your not DIY, it’s cheaper to have your mechanic just replace the fan motor with a new one.