Heater Problems

I have a 94 K1500 Silverado, and This last spring 07 the heater core needed to be replaced, started to leak on the passenger side, Well after that the heater has not blown out hot air since, just a luke warm feel only, and whats really funny is when you set the controls to floor or Def, it only blows outside cold air?????? I changed the Thermostate and still thats fine, also I changed the heater control button assy cause the lights went out but still having the same prob
Please help me on what you think it might be cause its the winter now and I like to drive it when its crappy out but dont like to freez my ass of HAHA.

Thank you.....:runaround:

Feel the inlet and outlet heater hoses, they should both be hot. If only the inlet one is hot then the heater core is probably plugged. If they are both hot then I would assume the heater core is installed improperly and isn't getting air routed like it is supposed to.

I have the EXACT same problem on my Grand Am. Somebody had replaced the heater core before I bought it and broke the HVAC box where those two straps screw on. I ended up duct taping the holes in the box on each side of the heater core, and it improved slightly. Is there a chance you broke the box on yours, or left the straps loose, letting air past the core?

If you find the solution for yours, please post the solution. I am going to replace the whole box on mine (looks like dash removal required), but I'm not sure if that will fix it.