Explorer running cold, how to fix?

I have a 1996 Explorer XLT V8. I got it in '05 from my mom who bought it new. It has 144,000 miles on it. Since I have had it, the engine has always run cold. In the summer this isn't a problem, but come winter it takes forever for the temp to come up enough for heat. When it does it is hot but not as hot as it should be. IMO
I have replaced the thermostat, and the heater core. Heater core was because it had a hole in it, not because it was running cold. I specifically got the factory thermostat to bring the temp reading back to normal, but it didn't work. My step dad told me that he had put a colder thermostat in it years back. Anyway, shouldn't be a problem with heater core sludge. Any ideas on what is going on? I don't want to spend another winter warming it up for 30 minutes to have heat.

Kebo

You have the infamous blend air door problem. You and all the other Explorer owners. Go to www.dormanproducts.com and enter all your vehicle information. Then go to the heater blend door category. There you will see an aftermarket repair kit to fix the problem. You can buy the kit at an autoparts store or order it online at www.rockauto.com