heater problem

help its cold outside :banghead: :banghead: i have an 88 f150 for some reason im not getting enough heat ive checked the heater core hoses and there both hot checked the cable that controls the heat thats working the door is too the thermastat seems to be working i think dont know if there is one in there if my heater core is slightly blocked what would be the best way to flush it and if it is the thermastat what would be the best way to replace it ive looked were its at and it looks kind of difficult to replace any suggestions would be very helpful besides getting rid of it lol:p

I've got an 89 with what sounds like a similar problem, what I discovered was the air flap behind the glove box and heater core cover was not sealing properly. The cold air from the outside was coming into the cab and through my defrost. I haven't corrected it yet because I am unsure about how to get into it so I can work on it. Hopefully this helps you.

figured out the problem heater core was slightly blocked and the inside flap was blocked with some clothes just got the truck two months ago so im slowly going through the small problems it has thanks for the info though it did help