the fan and ac doesn't work on the low settings only it will only work on the highest setting. so would it be the relay or the switch? how do you fix this?

 

The heater-A/C blower motor speed is typically controlled by a resistor pack that lowers the operating voltage. The pack is not connected at high speed setting, hence the fan has high speed operation only-failed resistor pack.

Location of the pack depends on year of car. It would be installed into the air ducting from the outside of the duct, the resistors exposed to the air stream to cool them.

Easy fix once you find the location.