Ford Repair: No heat, ford f150 4x4, core temp

Question
QUESTION: I have replace my heater core temp. switch and thermal relay on my truck and still i get no heat, any ideas

ANSWER: Dear Tommy, though I do not have enough knowledge of heating system, I think your heating box (element) maybe need to change.  You did not specify the truck make and model.  This would be my best guess.

---------- FOLLOW-UP ----------

QUESTION: Sorry it's a 1999 Ford F150 4x4  and I replaced that also

Answer
Tommy, thanks for the follow-up.  The last think that you could check is the vents and ducting system.  Maybe they're clogged.  Also, have checked if the radiator thermo. is in good working condition? Unless that what you meant by thermal relay....  If this true, then I have exhausted my limited knowledge of this subject.