No rear heat after front heater core replacement on 99 Expedition

Hopefully someone here can help me out. I just replaced the front heater core in my 99 Expedition. After I replaced the front heater core I noticed my rear heat doesn't heat right. Here are the symptoms. If I have the front heat running the rear heater doesn't heat. If i adjust the front air to cold than the rear heat starts to heat , but just a bit. I touched to two hoses going to the rear heater core an one is extremely hotter than the other. Did I miss something when I replaced the front heater core or did I do something wrong when I replaced the dash. Thanks for the help.

not sure of the replacement procedures, but is it possable you switched the heater hoses? the one"s that go into the heater core from the engine compartment? had this problem with an old ford truck i had, replaced the hoses in the engine compartment, could'nt get any heat to the cab, switched the heater core hoses, and almost ran me out of the cab! not sure why it made such a differance, but it did.
not sure if this will help you, but give it a try?
Rick, Ore