Do you have power to the switch and if so to the fan? The switch and fan are close together. You will have to lift the cab roof and remove the cover to access the fan and switch wiring. If you have juice to the fan then your fan motor is bad. If you have juice to the switch but none at fan then its the switch. If no power to the switch then you have to trace the wiring to the fuse box. I had a 4435 and it had a fuse box under the cab. A 4435 is a newer version of a 4425. I guarantee its fused somewheres, probably a cab power relay to. My 4435 owners manual had the locations of all the fuses and what they ran. You can go to the John Deere Parts website and pull up the wiring harness for the 4425, might help you trace the wiring and find the fuse box.