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.
Thanks for the reply; we did do all of the things you suggested and there is no power at fan or at switch. Cannot find a fuse to power the fan. There was a fuse labelled "cab" but that did not affect anything. All fuses were checked and found to be good, but still no power. There is a relay breaker in the motor compartment for the air conditioner according to the manual but still it seems to be working, too. I believe we have a wiring problem. Today I'll be taking the whole ceiling out, including speakers, etc., and try to trace the problem back on the wiring. There is power to radio, wiper, everything, but not to the fan. There is no power to the fan. The fan spins freely so it's not broken. Looked at relay that feeds switch and no power at relay.
Taking the interior out will not help, you will not gain access to the wiring. You need to lift your cab roof and remove the inner covers, they are bolted on. I pulled up the wiring harness on JDParts.com. The cab wiring harness runs down the right rear cab post and comes out under the cab and goes into the main junction box under the cab, same as my old 4435 did. Lots of fuses and relays in that box. If the fuses and relays are good you probably have a broken wire somewhere from the main fuse box under the cab up to the switch. Also make sure the fan itself is grounded. If you need the combine soon it might be easier just to run a wire up from another power source directly to the switch to provide power. Easier than tearing a wiring harness apart. I would also recommend you find a factory service manual, very handy as they have the wiring schematics. These old kraut combines are tough to trace wiring problems as the wiring is enclosed in plastic conduit. Good Luck!