Rob hit the nail on the head, more than likey. The power wire to the fan has 12 volts running to it... maximum. That powers the HIGH setting. In order to lower the fan speeds, the power goes through a resistor on the fan housing to get a lower voltage to power the fan for each lower speed setting. This resistor goes bad over time, leaving a full 12 volts to the fan at all times.
The resistor is usually on the fan housing for A/C cars.... under the dash for non-A/C cars.
It's possible, the resistor failed and the previous owner straight wired the fan so it only gets the full 12 volts.
Spenser seems to be a wiring guru... he's chimed in on this issue a time or two.
Here's one thread where he describes it's location and operation.
viewtopic.php?f=1&t=40721I typed in "fan motor location" in the search bar in the upper right corner of the screen.
It can use google to search posts on this site (really helpful, since if you only use the hbody site search function, you can only search a different topic every few minutes... something to do with spam attacks or something).... I was looking for a picture to assist.
But it's basically right on the fan housing under the hood.