you want the expansion of freon to happen just before entering the evap not 15 or so feet away from it. if the rear evap had it expansion valve all the way in front. the line between it and the rear evap would absorb all the outdoor heat and the rear evap would not work properly. When you put the it on the rear evap everything upstream is pressurized and hot. As soon as it goes thru the expansion valve pressure reduce freon get cold and the evap absorbs the surrounding heat. hence it gets cold and the blower blows air thru it to lower the temperature of the air.
Want a tech book recommendation just in case?
Tech book recommendation? Your post is my tech book recommendation! LOL Over the last few weeks my rear a/c was a little colder then my front. Now the front is cooler then the rear. This is definetely due to not having a rear expansion valve thats functioning properly. JMO