Why does the air in the rear never come on, or if it does it's hot not cold?

Hello, There may be one problem causing both issues, but there may be two problems as well. The rear unit uses the front air conditioner compressor, but has its own evaporator core, A/C lines, and expansion valve. If the front A/C is working properly, has not been serviced recently, and gets as cold as it has in years past, the problem is likely related to a blend door. The rear blend door and blend door actuator work to allow how or cool air to enter the ducts, or can block air from entering the ducts all together. If the actuator is bad or the blend door is physically damaged, it can cause the symptoms described. If the front A/C has been repaired or is not running as well as normal, you may have a bad expansion valve in the rear as well.