Those resistors are big and ugly, and spot welded to the Fan, replacing it alone is doable but a pain.
As you are saying it only works in high do you have a/c? as a 4 speed fan is wired different than a 3 speed.
If you have factory air, then verify that the motor connector (drivers side) of the heater box dosen't have any "burnt" pins.
You can even jumper 12V and ground to that plug to eliminate the switch and short cable itself.
If jumpering it at that connector fails, then your resistor is bad.
Getting to it is a pain, and involves careful prep. If your rain tray (under the plastic divertor) looks like the below:
Then there are a few screws that you have to remove to get the cover off.
Remove the Plastic Rain Divertor.
Remove the 2 Vacuum hoses from the outside, DON'T Pull them off....... Pry or cut them off as the plastic gets brittle over the years.
Remove the 2 Vacuum hoses from the inside of the car.
From inside the car.
On the passenger side (fresh air inlet)
Refer to this picture:
There are 2 screws that you need to remove to get the fresh/recirc divertor loose. Remove the Divertor off to the passenger side.
There is a bolt in the front of the fan cover that you remove from above, and a Phillips screw under the dash on the drivers side (long #2 Phillips screwdriver comes in handy).
Then remove the fan cover out on the Passenger Side.
Now you have the fan where you can see it.
See:
http://www.cabby-info.com/Files/ReplacingTheBlowerMotor.pdf
When replacing the airbox, go to a hardware store and get new closed cell foam to replace the old dead foam to Prevent water migration. Clean the drains in your air box too.