not that difficult to get the rear fans working, or to replace them if they are defective. I have seen many times, when the fans are stuck, that by removing them, and putting some oil on the shafts they will loosen up. You need to drip some light weight oil, such as 3 in 1, or sewing machine oil on the shafts, and then hold the motor so the oil runs down into the bearing, and manually turn the fan till it runs smooth when hand turning. It the rear bearing needs oil, sometimes there is a oil hole, or a tiny oil hole can be drilled in the rear of the case to get a few drops of oil to that bearing. Post some pictures of the fan motor, and let us know if there is a manufacturers name and / or a part number on the fan. You might even take the fan motor to a NAPA or Carquest to see if they can match it up. You need the rear fans to blow across the coil otherwise, the rear units are going to freeze up, and they might be damaged as a result.