Ford Repair: heat problem 97 sable, shade tree mechanic, frost plugs

Question
1997 sable, heat is only warm. New thermostat and car runs normal temp, mid-range on temp gauge. When turning temp knob on heater it changes from cold to warm and sounds like the blend door is working but it never gets hot. A/C works great. Flushed core and system sperately,it was nasty but runs free now(have replaced 2 frost plugs in the past year, so theres a problem); Core inlet hose gets hot, outlet hose from core is only warm.  What do you suggest?  I am a good shade tree mechanic but dont know Fords.  Do you think its the core?  Can I take off the cover and see blend door working? or Can I manually close the blend door all the way to check before I replace the core?

Answer
it's not blend door it is definitely heater core that is why one hose is hot and the other warm-causes are dirty cooling system and age-most of time it is customers fault for not maintaining cooling system but in your case it could be ford's fault also if you google 97 taurus brown coolant you will see what i am talking about-core will have to be replaced if you could flush enough to unclog it then it would probably leak-several hour job and if you do it make sure you have patience so you don't break anything.I know yours is a sable but more info out their on taurus and it is basically same car. good luck