The rear brake lights go through a module called the REM or (Rear Electronics Module). If the left brake light is working then more than likely it is the module. Both lights have the same input to the module, but they have seperate outputs. You will need to access the rem with a scan tool anf see if there are any codes. If there is you will need to do pinpoint tests on them. If there are none then the best thing to do is check the right brake light output at the connector and see if it goes out at the module if it does you need a new module. Either wy you need the Ford scan tool NGS or newer version. Most independent repair shops do not have this equipment. If you want to check the connector let me know and I will send you a pinout of the connector. The rem is located behind the trim panel on the right side in the rear. The removal of the entire rear quarter inside trim is required. I had the same problem on a 1999 windstar and found the module was bad. You need to store the information from the old module so you can load it into the new module when installed. You can try a used one but the van it came from needs to have the same accesories as yours. The rear climate control goes through the same module.
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