1991 Honda Civic Heater Core

i am needing advice on a heating issue in my car, the problem is my car doesnt produce enogh heat, when the car is sitting and warming up, the temperature gauge goes to the middle, but when driving in cold weather it goes down below the c and doesnt come back up but what i can figure out is when the gauge is down i still fell hot air, i thought it was the thermostat but i figure since it is hot that cant be it, i replaced the radiator, and thermostat once so the thermostat isnt very old, the only thing i havnt done is checked the heater core and removed and replaced that, the reason i have not is i dont know where it is located and i dont know how to remove it, can you help me with my issues and give me advice?[img:64136b22f1]http://www.2carpros.com/forum/automotive_pictures/163946_800px8891_Honda_Civic_Wagno_1.jpg[/img:64136b22f1]