replaced heater core in my 99 expedtion warm air in front cabin and cold air in the rear cabin.

just to help narrow down the options to my problem with my 99 expedtions heat. i noticed late last fall that my expedition wasn't blowing hot air in the front at all but hot air in the rear when turned on. my first thought was to start with the simplest and cheapest to fix. i replaced the thermostat first. it didn't help any. i then researched my problem online and checked my blend door and blend door motor next. it worked as it should. i crossed out any thoughts of the blower motor be faulty, its blowing hard enough on high it blow my cap off in the vent position. last but not least replaced the heater core. im now getting warm air out of the front vents but cold air out of the rear vents. got the coolant system to burp a few times by parking on a steep incline. but no difference was made to rear heat and still blowing warm air in front.. all out of ideas. any help  is greatly needed. thanks.