The blower fan turns on and blows air, but it turns on and off on its own?

This happens when the speed control module goes out which is common. Here is the location which is under the dash on the driver's side above the throttle, and the replacement instructions so you can change it out and fix the problem. Check out the diagrams (below). Please let us know if you need anything else to get the problem fixed.

Yes, from inside, you will need to look above the gas pedal to see if you can get your hands up there. There may be some items you will need to remove to gain access such as the lower dash panel. Check out what I mean below.