Pull it out and give it a good visual if its not too much trouble.The "older" ones Ive seen consist of 3 heavy wirewound resistors.The theory here is that the dash control "selects" which resistor to pass voltage through(this becomes the speed setting).Since you only have high,I suspect the common leg of these three resistors has come apart.High bypasses the resistor block....
You may be able to make a temporary fix by re-attaching that leg to its contact.
Concerning going to the dealer to get it...I suggest a call to NAPA.